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?"