The measure of a triangle is 180 degrees. In other words the sum of the angles of a triangle is 180 degrees
To solve for x set all of the expressions of angles equal to 180 and isolate x
11x - 3 + 7x + 5 + 13x - 8 = 180
31x + (- 6 + 6) = 180 + 6
31x/31 = 186 / 31
x = 6
To find the measure of each angle plug 6 into the x into each individual expression
Hope this helped!