We actually can use Sine, Cos, or Tangent to solve for the angles, which would give the angle, but graphing it came up with an Obtuse Scalene triangle.
If the painting lies in the middle of the rectangle and half of 20 f is 10 so then half of the painting that is on half of the wall would be only 1.2f if you take 10-1.2= 8.8f your final answer.