I don’t know what you are trying to do, but *= doesn’t appear to do what you think it does.
*= is an assignment operator. It evaluates its left-hand side and its right-hand side, multiplies them together, and assigns the result to the result of its left-hand side.
The result of a + b is not something to which you can assign. This is not valid code.
solved While solving the equation, why does it show an error for one variable ‘a’? [closed]