Answer
$C(x)=0.99x+10$
Work Step by Step
In a cost function of the form $C(x)=mx+b$,
m represents the marginal cost and
b represents the fixed cost.
b= $\$ 10$ (fixed cost) and
m= $\$ 0.99$ (the cost per downloaded song).
Let $x=$ the number of downloaded songs and
$C(x)=$ cost of downloading $x$ songs.
$C(x)=$ (marginal cost)(number of downloaded songs) + fixed cost
$C(x)=0.99x+10$
