The planet mars has two moons, deimos and phobos. phobos is about 9,000 km from mars and is about six times more massive than deimos. deimos is about 23,000 km from mars. upon which moon does mars exert a stronger gravitational force?

The gravitational force between two objects can be calculated through the equation,
                                  F = Gm₁m₂/d²
Where F is force, G is gravitational constant, m₁ and m₂ are the masses of the objects, and d is distance between them. 

(1) Gravitational force between Mars and Deimos,
                                  F = Gm₁m₂/(27000 km)²
(2) Gravitational force between Mars and Phobos,
                                 F = Gm₁(6m₂)/(9000 km)²

Evaluating the values above, we can then conclude that Mars exerts stronger gravitational force on Phobos than Deimos.