Please show all step. Thank you

13. The level curves of the function f (3:, y) = :1: - y, in the (33,11) plane (where :1: is on the horizontal axis) are all downward sloping. Can you give an explicit formula for a function g(:L', y) whose level curves are also downward sloping, but f latter than the level curves of f (3:, y)? (In other words, at every point in the plane, 9(33, y)'s level curve through the point should be atter than f (3:, y)'s level curve through that point)