Pay attention to:
1.nHoofs+nHorns<=k;
2.Use long or int arithmetics - not double or float
3.You have at first function of 2 variable. Try to transform it to the function of 1 variable and use simple forumla.
I think your know how to calc funciton's maximum :)
4.nHoofs and nHorns MUST be non-negative in this formula!

Good luck.

Solutions with float calculations are also easily acceptable. Use epsilons