Which formula do you use for this problem ?

two charged spheres, one suspended from a insulated thread and another held in place and unable to move.

when one sphere S1 is brought to a distance of 2.00 x 10^-2m from S2, the acceleration of S1 is 3.00 x 10^10 m/s^2. If S1 is brought to a distance of 4.00 x 12^-2 from S2, then the magnitude of the acceleration of S1 will be ???
What formula do I use???

Coulomb’s Law

F1=k•q1•q2/r1²
F2 = k•q1•q2/r2²,
Newton’s 2 Law
a1=F1/m = k•q1•q2/r1²•m
a2 =F2/m= k•q1•q2/r2²•m

a1/a2 ={ k•q1•q2/r1²•m}/ {k•q1•q2/r2²•m} =(r2/r1)²