You can start by plugging in values for #x#, multiply it by #-3# and subtract #3# to get the corresponding #y# value. This will give a table of values like this:
#{:("x","y"),(-2,3),(-1,0),(0,-3),(1,-6),(vdots,vdots):}#
You can then plot these points. You can plot any point #(x,y)#, by starting from the origin, #(0,0)#. The first number is the #x# value, which tells how far to move left or right. The second number is the #y# value, which tells you how far to move up or down.
All you really need is two points though. Connect the dots with a straight line and you have your graph. Notice that when the #x# values increase by one, the #y# values decrease by #3#. The amount of #y# change (or "rise") divided by the amount of #x# change (or "run") is called the slope of the graph, #m#, and is an important feature to help you understand how "steep" a straight line is.
Here is what that graph will look like when your done.
graph{y=-3x-3}
