Inverting matrix with unknown.

Hi all,

Quick question.

I'm just unsure of how to handle an unknown in a matrix when talking about finding the inverse.

Usually I simply augment (A|I) and reduce it to RREF and read the inverse off the identity matrix.

This is the matrix however I have to do it on -

$\displaystyle \begin{array}{cc}-2&1\\1&x\end{array} $

Obviously I'm not (in any way I can think of) able to reduce the x to a zero, so how do I transform this into reduced row echelon form, and hence get the inverse?

Cheers.