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Question Video: Finding the Slope and the 𝑦-Intercept of a Straight Line from a Given Graph Mathematics • 8th Grade

Find the slope π‘š and the 𝑦-intercept 𝑏 of this straight line.


Video Transcript

Find the slope π‘š and the 𝑦-intercept 𝑏 of this straight line.

First, let’s note what the slope is. Sometimes we say that the slope is the rise over the run, which is to say it’s the changes in 𝑦, the vertical changes, over the changes in π‘₯, the horizontal changes. And the 𝑦-intercept 𝑏 is the point where the line crosses the 𝑦-axis.

Using this diagram, we should be able to identify both the slope π‘š and the 𝑦-intercept 𝑏. We can calculate the slope, the changes in 𝑦 over the changes in π‘₯, between any two of the points on the line. For example, we could start at the point one, zero. And we could go to the point three, two. The rise, the changes in 𝑦, is positive two from zero along the 𝑦-axis to two along the 𝑦-axis. And the run, the changes in π‘₯ or the horizontal changes, are also positive two, from one to three along the π‘₯-axis.

Our changes in 𝑦 were positive two, and our changes in π‘₯ were positive two. Two over two is one, which tells us that our slope is one. We could confirm this. If we start at the point zero, negative one, we go up one and right one to get to the point one, zero. And one over one is again one.

Now, we want to consider the 𝑦-intercept, the place where this line crosses the 𝑦-axis. The 𝑦-axis is the vertical line. And this line crosses the 𝑦-axis here, at zero, negative one. The 𝑏-value will be the place along the 𝑦-axis this point crosses. And that is negative one.

You’ll probably remember that the general form for a linear equation is 𝑦 equals π‘šπ‘₯ plus 𝑏. Using this information, we could write an equation for this line, which would be positive one times π‘₯ minus one. But one times π‘₯ is just π‘₯. So, the equation for this line would be 𝑦 equals π‘₯ minus one. And under these conditions, the π‘š-value is one and the 𝑏-value is negative one.

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