Looking at the diagram, the box method has already been applied. The simplified answer would be gotten by adding up each term in the boxes while taking into consideration, the signs of each term. It becomes
- 15x - 15 - 12x^2 - 12x^3
Rearranging the terms in descending order of the exponents, it becomes
- 12x^3 - 12x^2 - 15x - 15 15x - 15 - 12x
