I'm assuming you know about matrix multiplication.
Set up your matrices first:
In the first one, your matrices are (note: you'll have to write these out to see them. ";" designates next row in the matrix)
[2,1,-2 ; 1,-2,-5 ; 4,1,7] * [x ; y ; z] = [7 ; -1 ; -1]
This can be simplified to a notation: A*B = C, where A, B, and C just represent the matrices I just showed you.
Since we want to find x, y, and z, we multiply both sides of the equation by A-1 (inverse matrix of A). This gives us: B = (A-1)*C
Wolfram alpha can help you at this point:http://mathworld.wolfram.com/MatrixInverse.html
The boxes within that big box are the determinants, which you will find how to solve here:http://en.wikipedia.org/wiki/Determinant#3-by-3_matrices
At that point, apply matrix multiplication to the 3x3 matrix A-1 and the 1x3 matrix C. Then incorporate the constant (which distributes to all the elements) and you'll have in each column of the result the value corresponding to the variable in the matrix on the other side of the equation.
I'm not sure how to solve the second one there, but I can tell you that each equation has four variables, two of which are always set to zero - so there slots in the matrix should be zero).
Written out (notice I've made "w" last):
[1,2,0,0 ; 3,0,4,0 ; 0,2,0,3 ; 0,0,3,-2] * [x ; y ; z] = [5;2;-2;1]
Hope that helps.
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