Question

What is the net gravitational field due the sun and earth at the center of the moon? You are given that the mass of the sun in 1.99 x 10^30 and the mass of the earth is 5.97 x 10^24. The distance from the center of the sun to the center of the earth is 1.496 x 10^8 and the distance from the center of the earth to the center of the moon is 3.9 x 10^5.
I do not know.