Chapter 1 - Graphs - 1.3 Lines - 1.3 Assess Your Understanding - Page 30: 8

$a=-\frac{1}{2}$

We know that two non-vertical lines are perpendicular if the product of their slopes is $-1$. The slope of the other line is $2$. Hence, the slope of the line perpendicular to it is: $2\cdot a=-1\\ a=-\frac{1}{2}$