Moderate# Finding a Base of a Trapezoid

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The area of a trapezoid is given by the formula $ A=\frac { 1 }{ 2 } h({ b }_{ 1 }+{ b }_{ 2 }) $, where $ h $ is the height and $ { b }_{ 1 } $ and $ { b }_{ 2 } $ are the two bases.

If one of the bases is $ 4 $ units long, then what is a correct formula for the other base?