Find the equations of the following ellipses: a) Center is (3, 7); one focus is (6, 7); one vertex is (8,7) b) Center is (4, -1); one vertex is (4, -5); one focus is (4,-3.5) c) Vertices are (5,9) and (5, 1): one focus is (5,7) d) Center is (5,6); the ellipse is tangent to both axises