it's a very straightforward problem, you only need to compute its determinant and set it different from zero.
I'll suggest some things before one of the wonderful members gives better advice.
You know that invertible matrices must have a non-zero determinant, so maybe calculating that can lead to some conclusions. The columns and rows must also be linearly independent, so I would think about that. Hope that helps some...
well if you're going to reduce row echelon form it, you can always set it up like:
And reduce that and you can see what values you need to set b and c to be, to make it invertible.
Though you automatically know that b and c cannot both be zero and otherwise it'd be not invertible.