paralell lines have the same slope
y=mx+b
m=slope
given
y=-1/3x+b
slope=-1/3
slope=rise/run
so from a given point, it goes down 1 unit for every 3 units it travels to the right
see which one does that
the slope is negative and should be sloping down, therefor we can eliminate A and C
B is going down 3 units for every 1 unit it travels to the right
D is the answer