Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$...\text{are parallel if } a=2$
RECALL: (i) Non-vertical lines are parallel if they have equal slopes. (ii) Non-vertical and non-horizontal lines are perpendicular if their slopes have a product of $-1$ Thus, for the given lines to be parallel, then $a$ must be $2$.