gimana nih caranya? minta bantuannya

∆PQR sebangun dengan ∆PST
PR/PS = PQ/PT
(12 + TR)/9 = 20/12
12(12 + TR) = 9(20)
144 + 12TR = 180
12TR = 36
TR = 3 cm

20/12 = (x+12)/9
5/3 = (x+12)/9
3(x+12) = 5.9
3x+36 = 45
X = 9/3 = 3