If f ( x ) f(x) is a linear function, f ( 1 ) = 3 f(-1)=3 , and f ( 3 ) =

If $f\left(x\right)$ is a linear function, $f(-1)=3$, and $f\left(3\right)=-2$, find an equation for $f\left(x\right)$

$f\left(x\right)$ =???