This document explains how to find the missing coordinate of a point on a line when the slope and one point are known. It provides examples of using the slope formula to solve for the missing x or y value. Given two points on a line and the slope, the slope formula can be used to set up an equation that can then be solved algebraically for the unknown coordinate. The document encourages practicing these types of problems and provides additional examples for the reader to work through on their own.

1. What’s the Point?
Find the missing coordinate

2. What if we have 2 points on the
same line, and we know the slope.
What do we do if one of the
points is missing a coordinate?
What then?
Image source: Thinkfutures.net
Well, we use the slope formula:
𝑚 =
𝑦 − 𝑦1
𝑥 − 𝑥1

3. 𝑚 =
𝑦 − 𝑦1
𝑥 − 𝑥1
We plug in for what we know and
solve for what we don’t know!
Let’s look at an example.
Suppose we have
(𝑥, 2), (3, −2), 𝑎𝑛𝑑 𝑚 = −1
𝑚 =
𝑦−𝑦1
𝑥−𝑥1
−1 =
2 − (−2)
𝑥 − 3
1. Plug in!
−1 =
4
𝑥 − 3
2. Simplify!
−𝑥 + 3 = 43. Cross multiply!
−𝑥 = 14. Subtract 3
from both sides!
𝑥 = −1
5.Divide both
sides by -1!

4. Let’s try another one!
m =
( , ) and ( , )
Let’s see if shapes
makes it any easier!
(𝑥, −3) 𝑎𝑛𝑑 (4,6); 𝑚 =
3
2
3
2
=
−3 6
𝑥 4
Plug in!
= −9
𝑥 − 4
Simplify!
3𝑥 − 12 = −18
3𝑥 = −6
𝑥 = −2
Cross multiply!
Add 12 to
both sides!
Divide both
sides by 3!

5. Now it’s your turn!
1. (−3,4) 𝑎𝑛𝑑 (𝑥, 5); 𝑚 =
1
6
2. 𝑥, 5 𝑎𝑛𝑑 2, −3 ; 𝑚 = −
4
3
3. 2,3 𝑎𝑛𝑑 𝑥, −2 ; 𝑚 = −
5
3
4. 𝑥, 3 𝑎𝑛𝑑 −2,1 ; 𝑚 =
1
3
Image source: MS ClipArt
Image source: www.flikr.com
Don’t forget – you can
use graph paper to help!!

6. Written by
Genny Phillips
March 23, 2015
Image source: cuentos-cuanticos.com