Since SP bisects angle RST, angles RSP and PST are both equal to each other and, when added together, add up to angle RST.
So to find the measure of RSP you could go through this process:
2(PST)=RST
2(y+5)=5/2y
((distribute))
2y+10=5/2y
((subtract 2y from both sides by making multiplying it by 2/2 and making it 4/2y)
10=1/2y
((multiply both sides by 2 to cancel out the 1/2 and get y alone on one side of the equation and your answer would be))
y=20
Now that you know the value of y, you can plug that into the measure of angle PST, which, as said before, is equal to angle RSP so you would have:
RSP=PST=y+5
RSP=PST=20+5
RSP=25
