Y=-99/4 when x=-11 and z=4. Find when x=2 and z=-8

The question can't be answered with the given information.

We don't know how y changes when x and z change.

That information would be given in the form of an 'equation',

of which there is none to be found here.

For example, the equation might be: y = 9x/z.

Then, as you said, y=-99/4 when x=-11 and z=4,

AND

when x=2 and z=-8, y would be - 2.25.

But, the equation might be y = (x-88) / z.

Then, y is still - 99/4 when x=-11 and z=4,

BUT

when x=2 and z=-8, y would be 10.75.

With only the information given in the question,

there's no way to be sure.