Question 897416
You plot the y-intercept, (0,b).
Then you use the slope, m, to calculate the next point. 
In your example,
{{{-2y=-120x+9000}}}
{{{y=60x-4500}}}
The slope, {{{m}}}, is equal to 60. 
The y-intercept is (0,-4500).
So for every 60 unit change in y, there is 1 unit change in x.
So if x changes by 10 units, y changes by 600 units.
So then another point on the line is (starting at the y-intercept),
(0,-4500)+(10,600)=(10,-3900)
You can then plot that point. 
Now you have two points, you can draw the straight line containing both points.