You can set up an equation for the large triangle made up of the smaller ones by knowing that the total interior angles must sum to 180°. We also can know that angle RPN is also 78° by the geometric law of alternate interior angles. Therefore you know that the angles to sum up are 2x, 2x (which comes from x + x), and 78°.
2x + 2x + 78 = 180
4x + 78 = 180
4x = 102
x = 25.5°
*note, there are many ways to solve this
