So we know that angle R=130 and because this is a parallelogram we can assume angle SPQ also equals 130 and knowing that we can find that angle RQP and RSP are equal both equal to 50 and since we know that RSP=50 we know SPT is also 50 because of alternate angles. SPT is an equilateral triangle so we also know that angle T is 50 degrees. All triangles degrees equal 180 so we can set up the problem the angle SPT(50) + the angle STP(50) + The angle PST(x) = 180 50+50=100 so it is 100+x=180 -100 PST(x)=80
