Explanation:
as simple as it sounds, we need to do a few steps to actually get the result.
first the outer angle 142° and the corresponding inner angle in the triangle are a linear pair of angles, and together they have 180°.
so, the inner angle is 180 - 142 = 38°.
then, remember, the sum of all angles in a triangle is always 180°.
so, the third inner angle is 180 - 38 - 68 = 74°
now, this third inner angle is with z (the corresponding outer angle) again a linear pair (sum 180°).
so,
z = 180 - 74 = 106°