Area = (base)*(height)
15 = (x+7)(x-7)
(x+7)(x-7) = 15
x^2 - 49 = 15
x^2 - 49+49 = 15+49
x^2 = 64
sqrt(x^2) = sqrt(64)
x = 8 ... note that x can't be negative
If x = 8, then the base is...
x+7 = 8+7 = 15
Therefore the base is 15 units.
(the height is x-7 = 8-7 = 1 unit)