If for a matrix, rank equals both the number of rows and number of columns, then the matrix is called

The matrix $$\left[ {\matrix{ 1 & { - 4} \cr 1 & { - 5} \cr } } \right]$$ is an inverse of the matrix $$\left[ {\matrix{ 5 & { - 4} \cr 1 & { - 1} \cr } } \right]$$

For the following matrix $$\left[ {\matrix{ 1 & { - 1} \cr 2 & 3 \cr } } \right]$$ the number of positive characteristic roots is

Given matrix $$L = \left[ {\matrix{ 2 & 1 \cr 3 & 2 \cr 4 & 5 \cr } } \right]\,\,$$ and $$M = \left[ {\matrix{ 3 & 2 \cr 0 & 1 \cr } } \right]$$
then $$L \times M$$ is