How do I work out if two lines are parallel?

The gradients of the two lines must be the same. The gradient is m in the equation: y=mx+c For example: L1: y = 3x – 2L2: 3y – 9x + 5 = 0 First rearrange L2 so it is in the same form as L1 to get:3y=9x-5Then divide L2 by 3 so the y coordinates are the same in L1 and L2 to get:y=3x-5/3Hence, the gradients are the same (3)