you need to rearrange your equation to make y the subject which would be
y= -2x+4
this now matches the formula y= mx+c , -2 being m and 4 being c.
c is your y intercept so the graph will go through 4 on the y axis. Then m is the gradient of your graph and because it is negative you know it is going downwards. 2 means that every unit value you go across the x axis the y axis will go down 2.
To find where your graph crosses the x axis you need to make y=0 in your equation which looks like
0=-2x+4
2x=4
x=2
Now you know that your x intercept is 2 so you can draw a line going through 4 on the y axis and 2 on the x axis like my photo.