The whole angle is 90 degrees right, so if you add the two angles that make up the whole angle, you’ll get 90 degrees. So all you have to do is add the measures and set them equal to 90. That’ll give you a simple equation to solve.
X+42+3x = 90
4x = 90-42
4x = 48
x = 12
I hope this helps you understand, if you don’t, let me know and ill see if i can clear anything up. ;)
