Slope is the amount that *y* changes, divided by the
amount *x* changes. That is, you pick two points on the line,
subtract the values of *y*, subtract the values of *x*,
and divide. If you do it right (and if your line is really a
line), you'll get the same answer no matter which two points you
pick.

In simple problems (like the ones in this section of the
website), you'll be given the two points as ordered pairs. For
example, you might be given (0, 3) and (2, −1). So the two
values of *y* are 3 and −1 and the two values
of *x* are 0 and 2. Subtracting the values of *y* gives
you 3 − (−1) = 4. Subtracting the valuesof *x*
gives you 0 − 2 = −2. Dividing gives you 4 /
(−2) = −2, so that's your answer.

Sometimes the two values of *x* are the same. Then their
difference is 0, which is bad because we can *never* divide
by 0. In that case, there is *no* slope, and you should say
so. (If you draw a line between the two points on graph paper, it
will be vertical.)

It doesn't matter what order you subtract the numbers in, so long
as you use the *same* order for *y* as you do
for *x*. In our example, say, we subtracted the first pair
minus the second pair for both *x* and *y*. If we had
done the two different ways around, we would have gotten a wrong
answer.