Answer
$r = 13.3~km$
Work Step by Step
We can find the volume of the sun after it collapsed:
$\rho = \frac{M_s}{V} = 2\times 10^{17}~kg/m^3$
$V = \frac{1.99\times 10^{30}~kg}{2\times 10^{17}~kg/m^3}$
$V = 9.95\times 10^{12}~m^3$
We can find the radius of the sun:
$V = \frac{4}{3}\pi~r^3 = 9.95\times 10^{12}~m^3$
$r^3 = \frac{(3)(9.95\times 10^{12}~m^3)}{4\pi}$
$r = \sqrt[3] {\frac{(3)(9.95\times 10^{12}~m^3)}{4\pi}}$
$r = 1.33\times 10^4~m$
$r = 13.3~km$
