Start with an empty graph. is already in (line)form. m is the slope, and b is the y intercept (what y is when x is zero). b is where you start on the y axis with your line, in this case 4. plot one point on (0,4). -2/3 is the slope. This is also known as rise over run. -2 is the change of the y axis, and 3 is the change on the x axis. It can also be that 2 is the change on the y axis and -3 is the change on the x axis. It doesn't matter, same line. I'll explain what I mean by "change" while we plot this line.
To plot the line, just start at (0,4), the point you've already plotted, and move in one of these two directions:
--to the right 3 and down 2 (over and over to plot points) or
--to the left 3 and up 2 (over and over to plot points)
These are the changes in the x and y axis. x moves right or left depending on whether the denominator of the slope is positive or negative, and y moves up or down depending on whether the numerator of the slope is positive or negative. (So if the slope is negative, the negative can go on either the denominator or the numerator.)
When you are done, your graph should look like this: