Since both triangles are similar, we know this because they have 2 angles in common, they both have the same third angle.
To find the third angle, we use the angle sum. The sum of angles in a triangle will always equal 180 degrees. We are given a right angle which is 90 degrees and another angle, which is 53 degrees. Knowing this:
90 + 53 + x = 180 (I have chosen to call the third angle x)
