Help on number 5 on page 69 please

Sure. The trick here is to look for opportunities to eliminate the variables you don’t want (i.e., y and z) by adding or subtracting. In this case, look what happens when you add all three equations together:

    \begin{align*}x+2y-3z&=92\\2x-y+z&=36\\4x-y+2z&=12\\\\7x+0y+0z&=140\\7x&=140\\x&=20\end{align*}