[Solved] Flexbox nested elements

body {
  background: skyblue;
}

a {
  text-decoration: none;
}

.wrapper {
  margin: 2%;
}

.articles {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -ms-flex-wrap: wrap;
      flex-wrap: wrap;
}

.article-card {
  background-color: #FFF;
  box-shadow: 0 4px 4px -4px rg;
}

.article-card--big {
  width: calc(50% - 30px);
  margin-right: 30px;
  float: left;
}

.article-card--flex {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -ms-flex-flow: row wrap;
      flex-flow: row wrap;
  width: 50%;
}

.article-card--landscape {
  width: calc(50% - 30px);
  float: left;
  margin-right: 30px;
  margin-bottom: 30px;
}
.article-card--landscape:nth-child(3), .article-card--landscape:nth-child(4) {
  margin-bottom: 0;
}

.card-content {
  display: block;
  padding: 15px;
}

.card-img {
  display: block;
  background: #ededed;
  position: relative;
  overflow: hidden;
}
.card-img:before {
  display: block;
  content: "";
  width: 100%;
  padding-top: 70%;
}
.card-img img {
  position: absolute;
  left: 0;
  right: 0;
  top: -100%;
  bottom: -100%;
  margin: auto 0;
  width: 100%;
  height: auto;
}

img {
  max-width: 100%;
  height: auto;
}

<div class="wrapper">
  <section class="articles articles-category">
    <article class="article-card article-card--big">
      <a href="">
        <span class="card-img"> <img src="http://www.placehold.it/300x240" alt=""> </span>
        <span class="card-content">
<span class="title">Dolores é Wyatt: teorias finais para o último episódio de Westworld</span>
        <span class="infos">
<span class="time">Há 21 horas</span>
        </span>
        </span>
      </a>
    </article>
    <div class="article-card--flex">
      <article class="article-card article-card--landscape">
        <a href="">
          <span class="card-img"> <img src="http://www.placehold.it/300x240" alt=""> </span>
          <span class="card-content">
<span class="title">Dolores é Wyatt: teorias finais para o último episódio de Westworld</span>
          <span class="infos">
<span class="time">Há 21 horas</span>
          </span>
          </span>
        </a>
      </article>
      <article class="article-card article-card--landscape">
        <a href="">
          <span class="card-img"> <img src="http://www.placehold.it/300x240" alt=""> </span>
          <span class="card-content">
<span class="title">Dolores é Wyatt: teorias finais para o último episódio de Westworld</span>
          <span class="infos">
<span class="time">Há 21 horas</span>
          </span>
          </span>
        </a>
      </article>
      <article class="article-card article-card--landscape">
        <a href="">
          <span class="card-img"> <img src="http://www.placehold.it/300x240" alt=""> </span>
          <span class="card-content">
<span class="title">Dolores é Wyatt: teorias finais para o último episódio de Westworld</span>
          <span class="infos">
<span class="time">Há 21 horas</span>
          </span>
          </span>
        </a>
      </article>
      <article class="article-card article-card--landscape">
        <a href="">
          <span class="card-img"> <img src="http://www.placehold.it/300x240" alt=""> </span>
          <span class="card-content">
<span class="title">Dolores é Wyatt: teorias finais para o último episódio de Westworld</span>
          <span class="infos">
<span class="time">Há 21 horas</span>
          </span>
          </span>
        </a>
      </article>
    </div>
  </section>
</div>