We have that the area is given by the equation
[tex]\begin{gathered} (x-\text{ 1)( x + 7) = 128} \\ x^2-x\text{ + 7x -7 = 128} \\ x^2+6x\text{ - 135 = 0} \\ (x\text{ - 9)(x +15) = 0 } \end{gathered}[/tex]So we have that x = 9 or x = -15, then we have that taking the positive x = 9.
