Guys I'm having a hard time solving this geometric problem given by my teacher

http://i324.photobucket.com/albums/k...i/untitled.jpg

Given : the figure is parallelogram

Line T to S = 42cm

Line A to C = 30cm

Line U to S = 21cm

I supposedly need to find the measure of Line B to S

but my teacher gave already the measure which is (Line B to S = 24cm)

but I need to know what is the procedure or formula on solving the measure of Line B to S

Could someone help me please?