The given triangle is a right angle triangle. This means that one of the angles is 90 degrees. Recall, the sum of the angles in a triangle is 180 degrees. This means that
2x + 15 + x + 90 = 180
2x + x + 15 + 90 = 180
3x + 105 = 180
3x = 180 - 105
3x = 75
x = 75/3
x = 25