we can use similar triangle method
PQ=8
QR=5
RS=15
ST=x+3
PR=RQ+QP
PR=8+5=13
RT=RS+ST
RT=15+x+3
RT=x+18
we know that smaller triangles and larger triangles are similar
so, we can find ratio of their sides

now, we can plug values

now, we can solve for x

we will get
x=21

so, option-B..........Answer