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answered 6 years ago
Rzyxp is a matrix, so Rzyxp[2] is its third row: a vector; albeit a one-dimensional one. In order to get the entry itself, you need to index with two indices:
Rzyxp
Rzyxp[2]
solve(Rzyxp[2,0],b)
does not give an error (but possibly not a very useful answer either).