#### Answer

$...\underline{\text{their slopes are equal}}$ and $\underline{\text{their y-intercepts}}$ are unequal;
$...\underline{\text{their slopes have a product of -1}}$.

#### Work Step by Step

The lines are parallel if their slopes are equal and the y-intercepts are unequal;
the lines are perpendicular if their slopes have a product of $-1$.