Hi there!
In this instance, the centripetal force experienced by the moon is equivalent to the gravitational force.
Thus:
[tex]\large\boxed{F_c = F_g}[/tex]
Centripetal acceleration, according to Newton's Second Law:
[tex]\Sigma F = ma \\\\F_c = m * a_c\\\\a_c = \frac{F_c}{m}[/tex]
Therefore:
[tex]a_c = \frac{F_g}{m_m} = \frac{2 * 10^{21}N}{1 * 10^{20}kg} = \boxed{20 N/kg}[/tex]