Given A and B, where A is a 3x3 matrix, and B is a vector of 3, and

AX=B

To solve for X, we would pre-multiply by the inverse of A to give

A^{-1}A X = A^{-1}B

Since A^{-1}A is the identity matrix by definition of the inverse, we end up with

X = A^{-1}B

To find X, it will be necessary to find the inverse of A, premultiply with B and the result (3x1 vector) will be X as required.

The inverse of A can be found by Gauss elimination, or any other method that you have learned.

The value of X is a multiple or sub-multiple of [17,4,7].