If y =12 when x= 6, find x when y=-24

We have no way of knowing how 'x' and 'y' are related, or whether
changing one of them causes the other one to change at all. 
There's no way to know that until you give us at least two points.

For example, let's say that 'x' and 'y' are the coordinates of points on a line.

-- If the equation of the line is [ x = 6 ], then it doesn't matter what 'y' is.
'x' is going to be 6 when y=12, also when y=-24, and also when y=4 million.

-- If the equation of the line is [ y = 2x ], then x=6 when y=12,
and x=-12 when y=-24.

-- If the equation of the line is [ y = 10x - 48 ], then x=6 when y=12,
and x=2.4  when y=-24.

You've told us only that the point (6, 12) is on the line.  There are an
infinite number of lines that can go through that point on the graph, and
every one of them has a different value for 'x' when y=-24 .