Answer
$300\;lb$.
Work Step by Step
The given values are
$F_l=7500\;lb$
$r_l=15\;in.$
$r_s=3\;in.$
Formula
$\Rightarrow \frac{F_l}{F_s}=\frac{r^2_l}{r^2_s}$
Substitute all the values.
$\Rightarrow \frac{7500\;lb}{F_s}=\frac{(15\;in.)^2}{(3\;in.)^2}$
Simplify.
$\Rightarrow (7500\;lb)\frac{(3\;in.)^2}{(15\;in.)^2}=F_s$
$\Rightarrow F_s=300\;lb$
Hence, the $F_s$ is $300\;lb$.
