Lines that are parallel to each other will have the same slope. Where slope, m, equals (y2-y1)/(x2-x1).
For line a:
Take two points on the line…
(x1,y1) => (0,3)
(x2,y2) => (-3,1)
m=(1-3)/(-3-0) = 2/3
For line b:
Take two points on the line…
(x1,y1) => (3,2)
(x2,y2) => (0,0)
m=(0-2)/(0-3) =2/3
For line c:
Take two points on the line…
(x1,y1) => (3,0)
(x2,y2) => (-2,-3)
m=(-3-0)/(-2-3) =3/5
Based off the information above, only lines a and b are parallel.