#### Answer

$C(x)=30x+60$

#### Work Step by Step

In a linear cost function, of the form $C(x)=mx+b$,
the $m$ represents the marginal cost and
$b$ represents the fixed cost (zero units produced).
The graph of C(x) passes through the points
$(8, 300)$ and $(0,60).$
Slope: $m=\displaystyle \frac{60-300}{0-8}=30$
Slope-intercept equation:
(b= the y-intercept)
$y=30x+60$
or
$C(x)=30x+60$