The area of a rectangle is the length times the width. The length is (2+x) and the width is (3+x). So the area of ABCD is...
Using the FOIL method of multiplying binomials, we get...
as the area of ABCD. However, the problem states that the area of ABCD is 9. Since both 9 and x^2+5x+6 are the area of ABCD, we can set them equal to each other.
Then, just subtract 6 from both sides, and you get...
which is the equation we were trying to get.