Answer:
-6
Step-by-step explanation:
You need to follow the order of operations. (PEMDAS)
Area of a rectangle is length by width, width is x, length is x+8
x×(x+8)=240
x²+8x=240
x²+8x-240=0
Use the quadratic formula to solve


It is a distance so can't be negative, so the answer must be 12
I order to do this question, you need to put the 13 on the right side. So now you have x^2+12x+0=13. Now, take half of 12, and square it. Add this to both sides to get x^2+12x+36=49. Now factor the left side by putting the half of 12 into this blank to get, (x+_)^2=49. Final Answer is,
(x+6)^2=49.