Expert: Scott A Wilson - 10/21/2009

Question
i can't fiqure out how to slove the problems that ask to Find the equation (in slope-intercept form). I get stuck at the ending when it asks to subtract the number on both sides. The fraction problems i can't fiqure out. Here's an example: (-1,-3), (2,-4. please help

Answer
The points given are (-1,-3) and (2,-4).

I will say that (x1,y1) = (-1,-3) and (x2,y2) = (2,-4).

It is known that m = (y2-y1)/(x2-x1), so here, m = (-4--3)/(2--1).
Note that -- is really +, so we have m = (-4+3)/(2+1) = -1/3.


The point-slope form is then y - y1 = m(x-x1).
That is, it's y - -3 = (-1/3)(x--1) = y + 3 = (-1/3)(x+1).
Multiplying out the function gives y + 3 = -x/3 - 1/3.
Subtracting 3 from both sides (and 3 is 9/3) gives
y = -x/3 - 1/3 - 9/3 = -x/3 - 10/3.
So the line is y = -x/3 - 10/3.

Now to go back and check to make sure this calculation was correct,
I'll check the first point and get y = 1/3 - 10/3 = -9/3 = -3, which is what was given.
For the second point, y = -2/3 - 10/3 = -12/3 = -4, so this point checks as well.