If the acceleration due to gravity at the surface of planet x is double the value of earth's, how does planet x's mass compare to earth's?

There is not enough information given to answer with. The force of gravity at the planet's surface depends on the planet's radius as well as its mass. The planet could have exactly the same mass as Earth has. But if it's radius is only 71% of Earth's radius, then gravity on its surface will be twice as strong as gravity on Earth.