Question

If y = 3 when X = -9 find X when y =5

Answer 1

We don't have enough information to answer that question.

You haven't told us anything about how 'y' depends on 'x',
or how 'y' reacts to a change in 'x'.

If we think about it in terms of a graph, you've given us one point
on the graph . . . the point  (-9, 3).  In order to answer the question,
we need to know something about the line that goes through the
point.  But you haven't told us anything about that, and there are
an infinite number of different lines that can go through one point.