Explanation:
the sum of all angles in a triangle is always 180°.
the triangle here has 2 known angles : 56° and 90°.
so, the third angle must be
180 - 56 - 90 = 34°
x is now the supplementary angle to this third inner angle of the triangle.
that means together they have 180°.
simply because the sum of all angles around a single point on one side of a line is always 180° (as this simulates a half-circle with the center at this point).
so,
34 + x = 180
x = 180 - 34 = 146°