Inside a uniform spherical shell,

Two satellites $A$ and $B$ move round the earth in the same orbit. The mass of $B$ is twice the mass of $A$. Which of the following statement is true?

Three particles each of equal mass $m$ are placed at three corners of an equilateral triangle of side $a$. Find the force exerted by this system on an other particle of mass $m$ placed at

$\left(a\right)$ the midpoint of a side.

$\left(b\right)$ the centre of the triangle.