Two lines are parallel if they have the same gradient. This can be found by looking at the coefficient of x. When the equation is written in the form y=ax+b, with b a constant, the gradient of the line would be a. So for the L1 the gradient is 3. So we want to get L2 in this form as well we rearrange L2, to 3y = 9x - 5, and then divide by 3 to get y = 3x -5/3. So the gradient of L2 is also 3 and therefore both lines are parallel.