I'm assuming that it's g/(b+v) and that this is simple algebraic isolation given that you've presented nothing else. If these assumptions are wrong, please provide more detail on what you're trying to accomplish.
1. Multiply both sides by (b+v)
R(b+v) = g
2. Divide both sides by R
b+v = g/R
3. Subtract b from both sides
v = (g/R) - b