Answer
$750$ units
Work Step by Step
Given,
$C(x) = 0.8x+900$
$R(x) = 2x$
To find break-even point,
$C(x) = R(x) $
$ 0.8x+900=2x$
$900=2x-0.8x$
$900=(2-0.8)x$
$900=1.2x$
$x = \frac{900}{1.2} = \frac{9000}{12} = 750$
$750$ units must be sold to break-even.