It's of the template y = mx+b
m = -8 = -8/1 = slope
b = 4 = y intercept
Start at the y intercept (0,4)
Move down 8 units and right 1 unit (due to the slope) to arrive at the point (1,-4)
Plot the points (0,4) and (1,-4)
Draw a straight line through them both. Extend the line as far as you can in both directions. GeoGebra and Desmos are two of many graphing tools.
Or you can use something like a TI83.