You might want to draw a picture to better visualize this, but you should come up with the equations.
length = 10 + 2x
width = 8 + 2x
You are trying to get the Area to equal to 120 sq yd.
You know area= length * width so substitute your equations and get:
120 = (10 + 2x)(8 + 2x)
now simplify:
120 = 80 + 36x + 4x^2
0 = 4x^2 + 36x - 40
0 = 4(x^2 + 9x - 10)
get to quadratic form:
0 = x^2 + 9x - 10
0 = (x + 10)(x - 1)
you get x=-10, and 1, obviously you cant have a negative answer, so x=1yard is the correct answer.