If Mars orbits the sun (radius=7 x10 to the 5th km) at a distance of 2.3 x10 to the 8th km,how does the size of this star during it's red-supergiant phase compare to the size of Mars' orbit?
Is the answer 9.24 x10 to the 6th?
This (and your previous similar question) are slightly unclear, as you don't specify what measure is to be used for "size". If it's the relative areas that you are looking for then that is: (7*10^5/2.3*10^8)^2 ≈ 9.26*10^-6 (note the -6, not one 6th).
If by "size" you mean relative radii, then you don't need to square the ratio.