Question

Suppose two sides of a square are given as x + 3 and 12 - x. What is its area?

Answer 1

The area is 15. Since squares have equal side lengths, you just have to find the value of x so that the answer is the same. x=4.5    
4.5 + 3 = 12 - 4.5 = 7.5     Then all you have to do is multiply it by 2. That will give you 15. I hope you understand.

Answer 2

Then you'd do this:

(x + 3)(12 - x)
= 12x - x^2 + 12x - 3x
= 21x - x^2

Hope it helped :)