The volumes will be the same, because the heights are equal and the cross-sectional areas at every level parallel to the bases are also equal.
Knowing the interior angle sum formula is;
180x(n-2)
180x(n-2)=1260 (solve for n here)
180n-360=1260
180n=1260+360
180n=1620
n=1620/180
n=9
Therefore, this polygon has 9 sides.
Hope I helped :)