First, find the angle of ∠RSP. ΔRSP is a triangle. Which means it's interior angles add up to 180 degrees. Find the missing length.
Let x = the missing length
79 + 43 + x = 180
122 + x = 180
x = 58
Now we know the angle of ∠RSP
Angles ∠RSP and ∠RST are supplementary angles. Which means they add up to 180 degrees.
We know the angle of ∠RSP
Find the angle of ∠RST
Let x = the missing angle
x + 58 = 180
x = 122
So, the answer is C 122 degrees.
