(Supposing angle RST measures 5x+15)Knowing that angle TQR is the opposite angle to RST the sum should be supplementary you can find the measures by adding the angle measurements.
5x+15+4x+3=180
9x+18=180
9x=162
18=x
if you plug in the x to the measure of RST (5*18+15)=105
the measure of RST would be 105 degrees