Explanation:
Consider the length of SB=
From similarity of the two triangles UTS & UCB
UT/ UC = US/ UB
6+18/6 = 12/UB
UB= 3
So BS = 12 - UB = 12 - 3 = 9 ###
Or just do it like that (another solution)
From similarity of the two triangles UTS & UCB
UT/CT = US/BS
18+6/18 = 12/BS
BS = 9 ###