The easiest way to draw the graph is to find the intercepts and draw a line through them.
x-intercept is found by putting y=0 and solve for x: 3x=21, x=21/3=7 is the x-intercept.
y-intercept is found by putting x=0 and solve for y: y=21 is the y-intercept.
So draw a line through (7,0) (on the x-axis) and (0,21) (on the y-axis).