Question 101200
This looks like the formula for the area of a trapezoid, so the {{{b[1]}}} and {{{b[2]}}} are the two bases of the trapezoid, so you can't really add them to get {{{b[3]}}}.  Otherwise, you have the right idea.
{{{A = (1/2)(b[1]+b[2])h}}} First, multiply both sides by 2.
{{{2A = (b[1]+b[2])h}}} Now divide both sides by {{{(b[1]+b[2])}}} to get:
{{{h = 2A/((b[1]+b[2]))}}}