Question

A=1/2h(b1+b2) solve for b2. How on earth do I do this?

Answer 1

To eliminate the fraction, multiply by 2 both sides of the equation. This gives,
                                     2A = h x (b1 + b2)
Then, divide both the left hand and the right hand side of the equation by the variable h.
                                 2A / h = b1 + b2
Next, subtract b1 from both sides,
                                (2A / h) - b1 = b2
Rearranging  the equation,
                                        b2 = (2A/h) - b1