check the picture below.
let's recall that the sum of all interior angles in a triangle is 180°.
so the green triangle's missing angle is 180 - 74 - 52 = 54.
the missing angle for the black triangle is 180 - 81 - 39 = 60.
so the tiny triangle's missing angle is 180 - 54 - 60 = 66 in yellow.
now, vertical angles, or namely angles across from each other at a junction like 66° and "x" are equal, thus x = 66°.
