You know that the sum of third angle of the triangle and the (2x-15) are supplementary. They add up to 180. So now solve for the third angle of the triangle. This will give 180-(2x-15). Now we know the sum of all the angles of a triangle = 180
35+ x +180-(2x-15)=180 distribute the negative
35+x+ 180-2x+15=180 combine like terms
-x=-50
X=50