Multiply the equation by the inverse of the coefficient of h. That coefficient is (b1 +b2)/2, so its inverse is 2/(b1 +b2). Parentheses are needed.
... 2A/(b1 +b2) = h
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<em>Comment on the answer choices</em>
As written, <em>none of them are correct</em>. The denominator can only be written without parentheses if the division bar is horizontal, and so serves also as a grouping symbol.
