Matrices Question

Matrices can be used to send coded messages. One way this is done is by taking a message, changing the letters into numbers, and then putting these numbers into a matrix. Zero represents a blank space. The matrix is formed by starting with the upper left most element, and going down each column till its full, then starting on the next column. If there are extra spaces in a matrix, fill the remaining elements with zeros (for example, if a message only has 7 letters, and one uses a 3-by-3 matrix, the remaining 2 elements would be zeros). Call the matrix containing the message matrix M. Then, one multiplies this by an encoding matrix; call this matrix A. Thus, the message sent would be the matrix AM. The recipient knows the encoding matrix, A. All they have to do is to multiply by the inverse, `A^{-1}`.

For the coded message `[[24,20,56,76,55,24,20,56,37],[87,78,206,267,185,87,78,206,127],[27,95,173,118,-110,27,95,173,1]]`, and the coding matrix `[[-1,3,4],[-3,10,14],[2,-6,7]]`, what is the message?

1. KEEP THIS TO YOURSELF ONLY
2. KEEP IT SECRET KEEP IT SAFE
3. DO NOT TELL ANYONE ANYTHING
4. KEEP CONCEALED AT ALL COSTS