[tex]\begin{bmatrix}x+2y=17\\ y-3z=4\end{bmatrix}[/tex]
[tex]\begin{bmatrix}y-3z=4\end{bmatrix}[/tex]
[tex]x=17-2\left(4+3z\right)[/tex]
[tex]x=-6z+9[/tex]
[tex]y=4+3z,\:x=-6z+9[/tex]
[tex]x\:+\:3y\:-\:3z=-6z+9+3\left(4+3z\right)-3z=-6z+9+12+9z-3z=21[/tex]
[tex](3x + 6y) + (2y -6z)=\left(3\left(-6z+9\right)+6\left(4+3z\right)\right)+\left(2\left(4+3z\right)-6z\right)=3\left(-6z+9\right)+8\left(3z+4\right)-6z=-18z+27+8\left(4+3z\right)-6z=-18z+27+32+24z-6z=59[/tex]
[tex]2x + 5y - 3z=2\left(-6z+9\right)+5\left(4+3z\right)-3z=-12z+18+5\left(4+3z\right)-3z=-12z+18+20+15z-3z=38[/tex]
[tex](2x \times 4y) \times (x + 4y - 6z)=\left(2\left(-6z+9\right)\times \:4\left(4+3z\right)\right)\left(-6z+9+4\left(4+3z\right)-6z\right)=8\left(-6z+9\right)\left(3z+4\right)\left(-6z-6z+4\left(3z+4\right)+9\right)=8\times \:25\left(-6z+9\right)\left(3z+4\right)=200\left(-6z+9\right)\left(3z+4\right)[/tex]
