= 6x² - 3y² + 1 in a: By setting one variable constant, find a plane that intersects the graph of z = (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.)