Answer
a) $C(m)=129.50+0.15m$
b) $562$ miles
Work Step by Step
a) Find a linear function that expresses the cost $C$ as a function of miles driven $m$:
$C(m)=129.50+0.15m$
b) The rental cost when 860 miles have been driven is $C(860)$:
$C(860)=129.50+0.15(860)=258.50$
c) Solve the equation $C(m)=213.80$:
$129.50+0.15m=213.80$
$129.50+0.15m-129.50=213.80-129.50$
$0.15m=84.3$
$m=\dfrac{84.3}{0.15}$
$m=562$
