Decide if the given level surface can be expressed as the graph of a function $f(x,y)$.

? Yes, it can No, it cannot  1.   $3 x + 6 y - 3 z - 8 = 0$

? Yes, it can No, it cannot  2.   $z^2 = x^2 + 6 y^2$

? Yes, it can No, it cannot  3.   $x^2 + y^2 + z^2 - z - 3 = 0$

? Yes, it can No, it cannot  4.   $z - x^2 - 9 y^2 = 0$