First put it in decreasing power according to x.
x^4-5x+8+x^2
=x^4+x^2-5x+8
Now this question is asking what is the highest degree of x you see in this function.
4 of course.
Therefore the answer to his question is degree of 4.
Done!
Where's the equation that contains "z"?
Elimination Method

If we multiply the equation 3 by (-1) we obtain this:

If we add them we obtain 0, therefore there are infinite solutions. So, let's write it in terms of Z
1. Using the 3rd equation we can obtain X(Y,Z)

2. We can replace this value of X in the 1st and 2nd equations

3. If we simplify:

4. We can obtain Y from this two equations:

5. Now, we need to obtain X(Z). We can replace Y in X(Y,Z)

6. If we simplify, we obtain:

7. In conclusion, we obtain that
(X,Y,Z) =