A*inv(B)*C form a square (6+3N) matrix but the submatrices have sizes:

A: (6+3N) x (6N)
B: (6N) x (6N)
C: (6N) x (6+3N)

The bulk of time taken to solve this sytem of equations is spent calculating the inverse of B. I know generally it is advised to avoid calculating inverses of matrices and transform the problem into a solution of a linear system. However, in this case, I don’t think it’s possible to do this? I just wanted a quick opinion to see if there’s anything clever I can do here. Cheers