The lines on two of the sides of the triangle indicate that the marked sides are equal to each other. This means that the angles across from the marked sides are equal as well. This means that you can make the side without anything labeled can be labeled as x as well. Make a formula with this knowledge:
90 + x + x = 180
You can now combine like terms:
90 + 2x = 180
Solve for x:
90 - 90 + 2x = 180 - 90
2x = 90
2x / 2 = 90 / 2
Hope this helped!
~Just a girl in love with Shawn Mendes