#-2x# is the gradient (or steepness) of the line. The gradient tells us that for #1# square to the right, how many squares we go up or down. With a negative number, we go downwards, and with a positive one we go up. this means that for every #1# square we go to the right, we go down #-2#.
#-4# is the y-intercept, which shows us at which point on the y-axis (vertical axis) the line crosses. Therefore, the line will cross the y-axis at #-4#.
to draw the line, plot a point on he coordinate #(0,-4)#, then go #1# box across and #2# down, and plot a second point. Continue to do this until you no longer can. Then work the opposite direction, going #1# to the left and #2# up