Graph the equation on the coordinate plane Y = -2x

This equation has the form y = mx + b.  Since b = 0, the y-intercept is (0,0).
The slope, m, is -2.

Plot the point (0,0).  Now, with your pencil on that point, move 1 unit to the right and 2 units down.  Plot a new point at that location.  Draw a straight line thru these 2 points.