Try this option (with short explanation):
15. m∠BAC=(180°-m∠ABC)/2=(180-60)/2=60°;
16. m∠SRT=(180-m∠STR)/2=(180-50)/2=65°;
17. if m∠TPR=60° and TP=RP=4, then m∠PRT=m∠PRT=m∠TPR=60°, then TR=TP=RP=4. Finally, TR=4;
18. if m∠ABC=m∠BAC, then AC=CB. Filnally, CB=3.
