Hi it's me again..

I'm having a bit of trouble visualizing this question. It's the last one in my "Holiday Homework package, and I have no idea what it means...

In any triangle ABC, prove that:

(a-b)/b= (2 sin(C/2) sin((A-B)/2))/ sinB

P.S: I didn't want to revive a dead thread, but I would like to thank Soroban for answering the question on my last thread!