Hey there!
I know you only asked for how to start it off, but I'll give you a walkthrough of the whole problem here.
The angles of a quadrilateral add up to 360º.
If I'm not wrong, this is a parallelogram, correct? That means there are two sets of equal angles.
∠TUR = ∠TSR while ∠STU = ∠SRU
Since ∠TUR = 138º, so will ∠TSR. Together they are 276º.
The other 2 angles have a combined measure of 84º because 360 - 276 = 84.
The remaining two angles are equal, so just divide 84 in half.
84 ÷ 2 = 42
Your answer is x = 42º, or option B.
Hope this helps!
