(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
