The sum of the angles in a triangle are equal to 180 degrees. Since we know the value of one angle is 90 degrees, subtract 90 from 180 to get 90. Now we know the sum of the other two angles must equal 90 degrees. If we add the x and 2x + 15 together, we get 3x + 15 = 90. Subtract 15 from both sides to get 3x = 75. Divide both sides by 3 to get x = 25.
