The first thing you need to figure out is the missing interior angle of the triangle.
There is 180 degrees in a triangle, there is a 90° and 52° angle listed, so subtract that from 180° to get the missing angle.
180° - 90° - 52° = 38°
Now we want the exterior angle x. Angle x and the interior angle form a straight line when combined which will also be a 180° angle. You can picture this as a half circle (a full circle is 360°).