Answer
To graph this by hand, first put your equation in y intercept form
y = mx + b
m = slope
b = y intercept
y = -1/2x + 3
Start with y intercept. Plot your y intercept of 3 on the y axis. This coordinate would be (0, 3) on the graph.
From there, use slope of -1/2 ( y/x ) to plot your mixture point. From y intercept of 3, move down -1 and right +2. Plot your point. Do this again, -1 down and right +2. Plot your point. Do this again so you have enough points to construct a straight line.
* note: sometimes you need points going the other direction for a straight line. You can do the inverse of -1/2 and instead do 1/-2 which is the same slope. Go up +1 and left -2. I did this in green on the attachment.