A black hole is an object whose gravitational field is so strong...

According to question the light is unable to escape the gravitational pull of the black hole thus the escape velocity is equal to the speed of the light so,
Escape velocity is ${V}_{e} = C$.
We know that escape velocity is given by,

$\sqrt{\dfrac{2GM}{R}} = 3 \times {10}^{8}$

Putting in the values we get,
$⇒ \sqrt{\dfrac{2 \times \left(\dfrac{20}{3} \times {10}^{- 11}\right)\left(6 \times {10}^{24}\right)}{R}} = 3 \times {10}^{8}$

Thus, on solving we get,
$R \approx 9mm \approx {10}^{- 2}m$