Why do you divide by 7

Is that what you mean?

7r = -14

Look, you need to get r alone (you need to solve for r, you need to find the value of r). This can be done by dividing 7r by 7. But in doing so, you have to divide -14 by 7 too.

\frac{7r}{7} = \frac{-14}{7}

Then, 7 divided by 7 gives 1.

1r = -2

And 1r = r

r = -2

If you use that value in the initial equation, you get:

7r = -14

7(-2) = -14

-14 = -14

which means that r has indeed the value of -2. :)