Explanation
The angles are: x, 66, 90
The three angles of any triangle always add to 180 degrees.
x+66+90 = 180
x+156 = 180
x = 180-156
x = 24
Or a slightly faster method would be
x+66 = 90
x = 90-66
x = 24
This shortcut is possible since the acute angles of a right triangle are complementary.
