Given the two functions $f$ and $g$, find $fg$ and identify the domain.
$f\left(x\right)=71x-2$, $g\left(x\right)=x+10$

$fg\left(x\right)=71x^2+712x-20$; Domain: $\left(-\infty,\infty\right)$

$fg\left(x\right)=71x^2-712x-20$; Domain: $\left(-\infty,0\right)$

$fg\left(x\right)=71x^2+708x-20$; Domain: $\left(-\infty,\infty\right)$

$fg\left(x\right)=71x^2-708x-20$; Domain: $\left(-\infty,\infty\right)$