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.

I’m sure you could google it. my profile picture might