Chapter F - Foundations: A Prelude to Functions - Section F.3 Lines - F.3 Assess Your Understanding - Page 29: 8

$...\text{are parallel if } a=2$

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 $2$.