it's a very straightforward problem, you only need to compute its determinant and set it different from zero.
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.