Let x be the original side of the square measure. Then from the condition, you have this quadratic equation for the new area (x+7)*(x-5) = 64. From this point, you can easily guess the solution x+7 = 16, x = 16-7 = 9 and then CHECK (9+7)*(9-5) = 16*4 = 64. Alternatively, you can solve equation (*) formally x^2 + 2x - 35 - 64 = 0 x^2 + 2x - 99 = 0 (x+11)*(x-9) = 0 etc.