For this case, the first thing to do is to observe that we are in the presence of an isosceles triangle.
We then have two equal sides:
RS = ST
Thus,
Angle R = Angle T
Then, we have the following equation:
5x = 180- (5x-20)
From here, we clear the value of x:
5x = -5x + 180 + 20
10x = 200
x = 200/10
x = 20
Answer:
the value of x is:
C. x = 20