Question

HOW DO I GRAPH in a rectangular coordinate system
(y=-2x)

Answer 1

your slope is -2. and your y intercept is 0. Start from the origin and do rise/run. Down two units, 1 unit to the right. Plot the point, repeat.

Answer 2

for every x you move forward, y moves down by two. Specifically find two points, i.e (0,0), (1,-2) put these point on a graph and draw a straight line through them. This will be your graph.