I need to solve this for b

Here's what I got so far:

a(a-b) = 3s(3s+b)

a^2 - ab = 9s^2 + 3sb

divide it all by a

a^2/ a - ab/a = 9s^2/a + 3sb/a

gives

a - b = 9s^s/a + 3sb/a

-b = 9s^s/a + 3sb/a -a

....not sure what to do about that b in 3sb?

Thanks