Answer
$SF=2.134\times10^8\text{ mi}$
Work Step by Step
It's easier to solve if there is a picture. In our picture the maximum distance of Mars from the sun is $SB=2.48\times10^8\text{ mi}$. Its minimum distance is $SA=3.46\times10^7\text{ mi}$. We have to find $SF$.
$SF=SB-FB.$ But from the properties of the ellipse, we know that $FB=SA$.
Hence $SF=SB-SA=2.48\times10^8-3.46\times10^7 (\text{mi})$
$SF=21.34\times10^7\text{ mi}=2.134\times10^8\text{ mi}$
