Solve recurrence relation merge sort

Question

I'd like to know how I can solve a recurrence relation like the one from merge sort. I know how to solve recurrence equations that start with $a(n)=a(n-1)+(n-1)$, but I don't know how to solve recurrence equations that start with $a(2n)$ like for example the equation of merge sort. How do I solve $a(2n+1)=a(n)+a(n+1)+2n$?