If y is 12 when x is 5, find x when y is 30

We have no idea how 'x' responds to changes in 'y'.
We don't even know their units, to make a guess with.

-- If 'x' and 'y' are the ages of two people, then when y = 30, 'x' will be  23.

-- If they are the frequency and wavelength of a wave on the lake,
then when y = 30, 'x' will be  2 .

-- If 'x' is the weight of a rock in your front yard and 'y' is the number of pages
in today's newspaper, then 'y' has no effect on 'x'.  No matter what happens
to 'y' . . . 'x' will always be  5 .

With the information given in the question, we just don't know.

We need either an equation, or else one more example.