If the area is 2x^3+30x^2+6x+90 and we know the width is x + 15, we can divide to find the length since area is length times width.
x goes into 2x^3 a 2x^2 number of times.
Multiply 2x^2 through by the x+15 dimension and we get 2x^3 + 30x^2.
Subtract from the initial dividend and we are left with the 6x+90 portion. x+15 goes into this 6 times. This means the final solution is 2x^2+6.