A Missing Edge Again

The surface area formula can run backwards like the volume formula. With the surface area and two edges known, it becomes an equation for the third edge.

This box has surface area 62 and a base 3 by 2. The rule says $62 = 2\cdot \left(3\cdot 2+3\cdot h+2\cdot h\right)$. Inside the bracket that is $6+5\cdot h$, so $62 = 12+10\cdot h$, then $50 = 10\cdot h$ and $h = 5$.

The unknown edge appears in two of the three pairs of faces, which is why it shows up twice in the bracket. Collect those terms before you divide.