What is the gravitational field strength at a height h above the surface of the Earth ? R is the radius of the earth.

Newtron's Law of Graviy is a good place to start - do you know that formula? The force acting on a mass by the Earth is equal to the gravitational field times that mass.

You know what gravity is at the face of the earth. You can find the mass with Radius R. As h increases, what happens and why?