don't know how to solve

Answer:
x=12
Step-by-step explanation:
Knowing that everything is given adds up to 90º(because the angle is a right angle), we can easily solve for x.
(x+42)+3x=90
Gather like terms.
4x+42=90
Subtract 42 from both sides of the equation.
4x=48
Divide 4 from both sides of the equation.
x=12
Hope this helps :)
Answer:
(x+42)°+3x°=90°
x+42°+3x°=90°
x+3x°+42°=90°
4x°+42°=90°
4x°=90°-42°
4x°=48°
x°=48°/4
x°=12°