#### Answer

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

#### Work Step by Step

When a linear equation is written in slope-intercept form, or y=mx+b form, m=slope and b=y-intercept.
The equation y=2ax+4 is written in y=mx+b form and has a slope of 2a.
Since m=-1, we can solve for a now that we know m=2a because if
m=-1, and
m=2a, then
-1=2a
Now, to solve for a, we divide by 2 on both sides
$\frac{-1}{2}$=$\frac{2a}{2}$
a=-$\frac{1}{2}$