Here is an example of another problem. Its is simalar to this one.

* Find the equation of the line that passes through the points (–2, 4) and (1, 2).

Well, if I have two points on a straight line, I can always find the slope; that’s what the slope formula is for.

See this image.

http://purplemath.com/modules/slope/slope02.gif

slope m = -2/3

Now I have the slope and two points. I know I can find the equation (by solving first for “b”) if I have a point and the slope. So I need to pick one of the points (it doesn’t matter which one), and use it to solve for b. Using the point (–2, 4), I get:

y = mx + b

4 = (– 2/3)(–2) + b

4 = 4/3 + b

4 – 4/3 = b

12/3 – 4/3 = b

b = 8/3

…so y = (– 2/3)x + 8/3.

On the other hand, if I use the point (1, 2), I get:

y = mx + b

2 = (– 2/3)(1) + b

2 = – 2/3 + b

2 + 2/3 = b

6/3 + 2/3 = b

b = 8/3

…so y = (– 2/3)x + 8/3, just as when I used the other point.

As you can see, once you have the slope, it doesn’t matter which point you use in order to find the line equation. The answer will work out the same either way.