Google says: http://mdm4u1.wetpaint.com/page/Enco...using+matrices

you can break the message into a series of 2x2 matrices and multiple each by theinverseof the coding matrix.

This means you need to know how your message string is supposed to be broken up into 2x2 elements. Presumably your course teacher or the question will have told you the notation for this (ie, wheter each pair of numbers in the message corresponds to a row or a column).

I cant follow your working, but if

is supposed to be the inverse of the coding matrix it doesn't appear to be correct. see link| -3 1 |

| 2 -1 |

http://www.wolframalpha.com/input/?i...}%2C+{1%2C+3}}