Two lines are parallel when they have the same slope.
For example, the lines y = 2x + 3 and y = 2x + 5 are parallel, because both of them have the slope = 2.
So if the slope of one line is m1 and the slope of the second line is m2, in order to these lines be parallel, we have that m1 = m2 (same slopes)
So the answer is "m1 = m2"
