Make b the subject of the formula:

s=ab-bc

Here, you just want to solve for b. because the right side of the equation doesn't have any like terms, you have to factor that b out:

s = ab - bc
s = b(a - c)

divide both sides by (a - c)

s/(a - c) = b is your answer. it's "s" over "a - c" if the formatting is difficult to read

S=ab-bc
factorise it:
s=b(a-c)
divide by (a-c) to isolate 'b':
2/(a-c)=b

Hope this helps :)