All the corners of a rectangle is equal to 90 degrees. So you need to set (x-1) and (2x+10) equal to 90 and then solve for x.
(x - 1) + (2x + 10) = 90
Combine the like terms...
3x + 9 = 90
Subtract both sides by 9...
3x + 9 - 9 = 90 - 9
3x = 81
Divide both sides by 3...
3x = 81
(3x) / 3 = 81 / 3
x = 27