That’s because although you call the function you don’t save its return value anywhere. Try instead
def main():
price = equation(5, 10)
print(price)
Note also that the variable in the calling function doesn’t need to relate to naes inside the called function (which are generally only available inside the function). So you could equally well write
def main():
some_other_name = equation(5, 10)
print(some_other_name)
solved Simply use variables throughout functions