b needs to be transposed in the following:

a = sqrt(b / b+c)

It will become:

a^2 = b / b+c

Then I can cancel out b+c by multiplying it on both sides:

a^2 (b+c) = b / b+c x b+c

b = a^2b + a^2c

^ that's what I get, but it isn't the answer .. Help please