ShmoopTube

00:03

Here’s an unshmoopy question you may find on an exam somewhere in life…

00:07

A line parallel to another line whose graph passes through the point (3, 8) and has a y-intercept

00:13

of -1 must have a slope of… what?

00:17

And here are the potential answers…

00:22

So… what is this question asking?

00:23

How well do we really know the point slope formula, y equals mx plus b?

00:29

First of all, this whole “line is parallel” business is just the test-makers trying to trip us up.

00:35

All parallel lines have the same slope, so let’s not fall prey to their wily schemes.

00:39

So in the y equals mx plus b format of describing a line, we have b already – it’s negative 1…

00:46

the problem tells us that much.

00:48

So we now just have to solve for slope by taking what we know and plugging it into the base equation.

00:53

Well, b is negative 1 so it’s:

00:56

y equals mx minus 1

00:58

But the problem GIVES us the point 3, 8, which we know the line passes through…

01:03

so we can simply plug in those points – 3 is x and 8 is y.

01:07

We get 8 equals m times 3 minus 1

01:12

Or, by adding 1 to each side, we have 9 equals 3m.

01:16

Our slope is 3… so our answer is C.

Full Transcript