Can someone help me figure out what formula I am to use for this word problem? I am having a hard time.

Question

If the sides of a square are lengthened by 7cm the area of the square becomes 121cm^2. What is the length of a side of the orgional square?

Answer 1

(x+7)^2 =121
x+7 = +/- 11
x = 3 or -18
-18 will not do so use 3

Answer 2

Thank you!