I'm not quite sure how to get started on this one, any suggestions? Thanks!!

"If you have the following:
R-squared for Y on X1,2,3 is 0.25,
R-squared for Y on X1,2 is 0.15,
the variance of Y is 100,
and there are 50 cases,
then what would the incremental explained sum of squares for X3 net of X12 equal?"