A lesson on finding a missing coordinate using the slope.

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