The equation y = -3x-3 is of the form y = mx+b
m = -3 = slope
b = -3 = y intercept
Start at (0,-3) which is where the y intercept is located. Then move down 3 and right 1, due to the slope. This moves us from (0,-3) to (1,-6)
Plot the two points (0,-3) and (1,-6). Then draw a straight line through them. Extend the line as far as you can in both directions. This completes the graph of y = -3x-3
The graph of y = -3 will be a horizontal line through -3 on the y axis. Think of it as y = 0x-3.
The graph is shown below.