Answer
$y=-0.5x$
Work Step by Step
The slope of the line can be found following the equation below:
$m=\frac{y_2-y_1}{x_2-x_1}$ where x and y are the coordinates of the points on the line, here: $(0,0)$ and $(-2,1)$.
$m=\frac{1-0}{-2-0}=\frac{1}{-2}=-0.5$
If we have the slope and a point on the graph (now, we can take $(0,0)$), we can use the point-slope formula to calculate the equation of the line:
$y-y_1=m(x-x_1)$
which translates to $y-0=-0.5(x-0)$
The equation of the line is therefore: $y=-0.5x$
