Pull out first x^2 ,so it becomes 1+x^2+.....
The outside x^2 becomes x^8 because of power 4,subtract this from x^12,...
So we need to find coefficient of x^4 here
Since this series is has even numbers ,it clearly says it has x of the form x^2r. ,And we are finding x^4 and thus r=2
Here n= 4,r=2 hence ,(4+2-1,2)=10