Question

If y=-3 when x=15, find y when x=-2?

Answer 1

You haven't established a relationship between x and y here.

If you were to plot a point on a graph at (15,-3), then a second point at x-2 could be placed anywhere along the y axis.

IOW, y could be any number.

Answer 2

Let me show you what I mean:
If y = x-18, then y would be -20 when x was -2

If y = x/-5 then y would be .4 when x was -2

In both cases above, y would be -3 when x was 15