Your first step should be to make B1 not be in the denominator of a fraction. Either take the reciprocal of each side or multiply both sides by that denominator. You could multiply both sides just by (B1 + B2) also. In the end, you'll end up with the same answer regardless.
edit: I'm assuming 2(b1 + b2)h are all in the denominator, though it's a little ambiguous the way you've written it.