I have the following communication channel represented with its transitional matrix:
and I'm supposed to calculate its capacity.
I'm thinking it's probably quite simple, but nothing comes to mind at the moment. It's not a symmetric channel so I don't really know what to do.

Hint:
Can you compute the capacity of the channel given by the upper left $3\times 3$ submatrix?
Can you compute the capacity of a channel composed by $2$ (or in general, $n$) subchannels "in parallel" (one and only one of the subchannels can be used at each channel trasmision) where the input-output alphabets of the subchannels don't overlap?