Whitney, whenever you are wanting the equation for a straight line, you need to find two things: How steep the line is (the slope) and its height when x=0 (the y-intercept). In the end, there a few ways to represent this information, but a typical answer will look something like y = mx + b.
Point-slope form is one approach for finding a line and it requires two things also: A point and a slope. Pretty sweet name, huh?
1. A point. It looks like they gave you two to pick from here. Pick either (2,3) or (5,1). It doesn't matter which; the answer will come out the same.
2. A slope. Well, this question doesn't tell you the slope, so we'll need to calculate it. Slope is calculated by the "rise-over-run" equation. The rise is how much the y-coordinates changed, the run is how much the x-coordinates changed. Looking at (2,3) and (5,1), how much did y change? It went from 3 to 1, so it changed by -2 (it went down 2). How much did x change? It went from 2 to 5, so it changed by 3 (it went up 3). The slope is the change in y (rise) divided by the change in x (run). The previous post has already done the arithmetic for you, but I'm sure you can do that on your own, too!
3. Plug-n-chug. Point-slope form looks like this:
y - y1 = m(x - x1)
The y1 and x1 are where you put the point that you picked in step one (remember that either point will be fine). The m is the slope that you calculated in step 2.
At this point you are done! Awesome! You have it in point-slope form, which is what the question asked you to do.
Extra Credit Reading
4. Slope-intercept form
But, could we do better? Probably. Read the assignment heading carefully and see whether it asks you to convert your answer to slope-intercept form.
If it does, then there is one more simple step. Slope-Intercept form is just another way to express the details of a line (kind of like having more than one word that means "awesome"). You need two things to use this form (you guessed it! A slope and an intercept). Right now, you have something that looks like this:
y - y1 = m(x - x1)
All you want to do is have it look like this instead:
y = mx + b
Do you see that they both have an m? That means that number will stay the same. So will y and x for the same reason. To get b, all you do is take your point and slope and use this equation:
b = y1 - mx1
Fill in the x1 and y1 with the point you picked in step 1 and m with the slope you calculated in step 2. Do the arithmetic, and this will give you the value of b.
5. It really doesn't matter which point I pick?
Nope! Try both, and you'll see that they both have the same slope-intercept form.