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.