Answer
$y=-10e^{-\frac{x}{2}} +C$
Work Step by Step
Use separation of variables, then integrate the differential equation.
$\frac{dy}{dx}= 5e^{-\frac{x}{2}} $
$\int dy= \int 5e^{-\frac{x}{2}} dx$
Use u-substitution
let $ u=-\frac{x}{2}$
then $du= -\frac{1}{2} dx$
$y=-10\int e^u du$
$y= -10e^{-\frac{x}{2}} +C$