Answer:
x = 40°
Step-by-step explanation:
Given that ∠SRT ≅ ∠STR
Then ∠STR = 20
∠STR and ∠STU are what you call supplementary angles. This means that the sum of their measurements is equal to 180° because they form a straight line.
So if:
∠STR + ∠STU = 180°
m∠STU = 4x
m∠STR = 20
Then:
4x + 20° = 180°
Now we solve for x:
x = 40°