Assume that y varies directly with x, then solve.
If y=12 when x=15, find x when y=21.

The answer is in a fraction!
it is x = 105 / ? , I cant figure out the bottom .

We, hold up there for just a second. 

"A plane" is infinite.  Whatever direction you pick in "a plane",
the plane keeps going in that direction forever and never ends.
So "a plane" has NO dimensions.

Any PART of "a plane" has TWO dimensions, because any PART
of it that you pick occupies some length and some width.

A better way to say it is like this:

If you stick a nail into the plane at one point, THEN, to describe ANY
OTHER point in the plane relative to the nail, you need TWO NUMBERS.

THAT's why a line has one dimension, a plane has two, and a solid has
three dimensions ... it's how many numbers you need if you want to
send somebody to another point from where you're sitting.