By setting one variable constant, find a plane that intersects the graph of z = 3y² – 9x² + 5 in a: (a) Parabola opening upward: the plane (Give your answer by specifying the variable in the first answer blank and a value for it in the second.) = (b) Parabola opening downward: the plane (Give your answer by specifying the variable in the first answer blank and a value for it in the second.) = (c) Pair of intersecting straight lines: the plane (Give your answer by specifying the variable in the first answer blank and a value for it in the second.)