Solve the system.
1/4x+1/3y+1/2z=-7/12
1/3x+2/3y+1/4z=-17/3
1/2x-1/3y+2/3z=19/6
It might be easier to clear the fractions first:
3x+4y+6z = -7
4x+8y+3z = -68
3x-2y+4z = 19
Now you can use substitution or elimination, or matrices, or determinants, depending on what you have studied. Follow the steps in your text...