Answer
$2.96Km$
Work Step by Step
We know that
$V_{esc}=\sqrt{\frac{2GM_S}{R}}$
as $V_{esc}=c$, where c is speed of light
$\implies c=\sqrt{\frac{2GM_S}{R}}$
This can be rearranged as:
$R=\frac{2GM_S}{c^2}$
We plug in the known values to obtain:
$R=\frac{2(6.67\times 10^{-11})(2.0\times 10^{30})}{(3.0\times 10^8)^2}=2.96Km$
