we now know that the slope is -2 and since parallel lines have the same slope we know the line we are getting the equation to has a slop of -2. From here there are 2 ways to do this problem we can either use point slope formula or sub in the point and slope into the eqation y=mx+b and solve for b.
Using point slope formula
y-y1=m(x-x1)
where y1=1, x1=3, and m = -2
y-1=-2(x-3)
y=-2x+6+1
y=-2x+7
using algebra
y=mx+b
where y=1 m=-2 and x=3
1=-2(3)+b
1=-6+b
7=b
y=-2x+7