# Simple Math Question

**URL:** <https://boards.straightdope.com/t/simple-math-question/391661>\
**Category:** Factual Questions\
**Created:** [February 10, 2007, 10:07pm UTC](https://boards.straightdope.com/t/simple-math-question/391661 "2007-02-10T22:07:19Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Khadaji](https://avatars.discourse-cdn.com/v4/letter/k/9e8a1a/32.png) [@Khadaji](https://boards.straightdope.com/u/Khadaji)\
**Post date:** [February 10, 2007, 10:07pm UTC](https://boards.straightdope.com/t/simple-math-question/391661/1 "2007-02-10T22:07:19Z")

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OK, I am embarassed to admit that I am having a brain freeze and cannot see this answer. I know I was taught this once. ☹

I have two points. X1, Y1 and X2, Y2.

I have the X value of the third point. X3. It falls between X1 and X2.

How do I figure out Y3?

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**Author:** ![Tangent](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tangent/32/3098_2.png) [@Tangent](https://boards.straightdope.com/u/Tangent)\
**Post date:** [February 10, 2007, 10:13pm UTC](https://boards.straightdope.com/t/simple-math-question/391661/2 "2007-02-10T22:13:02Z")

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Are the points all supposed to fall on a straight line?

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**Author:** ![Rysto](https://avatars.discourse-cdn.com/v4/letter/r/ecccb3/32.png) [@Rysto](https://boards.straightdope.com/u/Rysto)\
**Post date:** [February 10, 2007, 10:23pm UTC](https://boards.straightdope.com/t/simple-math-question/391661/3 "2007-02-10T22:23:12Z")

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Assuming that all of the points fall on a straight line:

The slope of this line, m, is (y2 - y1)/(x2 - x1)  
The y-intercept of the line, b, is y2 - m \* x2.

The equation for this line is:

y = m \* x + b

So y3 = m \* x3 + b

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**Author:** ![Khadaji](https://avatars.discourse-cdn.com/v4/letter/k/9e8a1a/32.png) [@Khadaji](https://boards.straightdope.com/u/Khadaji)\
**Post date:** [February 10, 2007, 10:26pm UTC](https://boards.straightdope.com/t/simple-math-question/391661/4 "2007-02-10T22:26:27Z")

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Thanks. Yes, all on a straight line - my apologies for asking the question so poorly and my thanks for helping with the answer!