y + 3z = 7 + 4x
2y + x = 1 - 5z
6x - 9 = 2y + 3z

First arrange terms in alphabet order on the left
and constant terms on the right, like this:


Put all the coefficients in a 3x3 matrix,
multiplied by a 3x1 matrix of unknowns or 
variables, and set that equal to the 3x1
matrix of constants on the right:



That's  form

because , 
, 


Edwin