Calculate the y value for a bunch of x values.
E.g. x=0: y=-5*0 = 0, plot the point (0,0)
x=1: y=-5*1=-5, plot the point (1,-5)
repeat the process until you are confident to draw a smooth line through it. For straight lines two points suffice. In this example I'd choose them sufficiently far apart, like x=-4 and x=+4.