There are three unknowns and only two equations. However, we can find two variables in terms of the third. The second equation gives x=7-2z so we can substitute for x in the first equation: 14-4z+y-z=4; y=5z-10 or y=5(z-2). There are many solutions and one way of showing them is to draw the two equations on the same graph. Make z the horizontal axis. The vertical axis serves for x and y provided we label the lines after drawing them. The z intercepts are at 3.5 and 2, and the x and y intercepts are at 7 (for x) and -10 (for y), both marked on the vertical axis. Draw a line through (0,-10) and (2,0) for y=5(z-2); and a line through (0,7) and (3.5,0) for x=7-2x.
A vertical line through the z axis at a particular value of z will cut the two lines and show what the values of x and y are for that particular value of z.