This is a linear (straight line) graph so only two points need to be found. The line drawn through the points is the graph. The best points to choose are the intercepts.
To find the x-intercept, put y=0 and solve for x: -x=-9 so x=9, making the intercept (mark the x-axis) the point (9,0).
The y-intercept is found by putting x=0 and solving for y: -3y=-9, so 3y=9 and y=9/3=3 (mark the y-axis), making the y-intercept (0,3).
Draw a line through (9,0) and (0,3). y is the vertical axis and x is the horizontal axis. Marking the intercepts on the axes gives you an idea of the scale of the graph. Always allow enough room when measuring out and positioning the axes.
Note that -x-3y=-9 is the same as x+3y=9.
