According to the Universal Law of Gravitation, any two bodies exert a gravitational force on each other and this force is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Mathematically,
`F = (Gm_1m_2)/(r^2)`
where, F is the gravitational force, G is the universal gravitational constant, m1 and m2 are the masses of the objects and r is the distance between them.
In the given case, m1 = m2 = 0.879 kg; F = `8.04 xx 10^(-11)` N and
G = `6.673 xx 10^(-11) Nm^2 /(kg)^2`
Substituting the value of these variables into the equation, we get,
`r^2 = Gm_1m_2/F`
`r^2 = (6.673 xx 10^(-11) xx 0.879 xx 0.879)/(8.04 xx 10^(-11))`
Solving this equation, we get, r = 0.8 m
that is, the two balls are 0.8 m away from each other.
Hope this helps.