Answer
The expression for the missing side
$ = 10x-3$.
Work Step by Step
Let $y = $ the expression for the missing side
$(4x-7)y = 40x^{2}-82x+21$
$y = \frac{40x^{2}-82x+21}{4x-7}$
$= \frac{40x^{2}-70x-12x+21}{4x-7}$
$= \frac{10x(4x-7)-3(4x-7)}{4x-7}$
$= \frac{(10x-3)(4x-7)}{4x-7}$
$= \frac{(10x-3)}{1}$
$y = 10x-3$
