The best I can do for you here is to assume that y ou're supposed to graph y=a.5x-2. This could be re-written as y = 0.5x - 2 (letting a = 1).
This is a linear function with slope m = 0.5 and y-intercept (0, -2).
If a = 2 (which is just an example), then y = 0.5(2)x - 2. This has a slope of 1 and a y-incept of (0.-2).
If this is not what you wanted, then please put more effort into presenting your question and the directions that came with it. Thanks.