So first you would have to find the slope using the two points and the equation: y2-y1/x2-x1, which in this case means you take 6+4/3+1, which would be a slope of 10/4 or 5/2, or 2.5. Using this, we can take a point, use point slope form, and rearrange it. So:
y-6=2.5(x-3)
y-6=2.5x-7.5
-2.5x+y=1.5