Determinging Gravity

Sorry.. I think I posted this post already but I can't find the original thread..

Problem:
Two spheres of masses m = 1.00 g and m' = 1.00 * 10^2
kg are isolated from all other bodies and are initially at
rest, with their centers a distance r = 15.0 cm apart. One
minute later, the smaller sphere has moved 0.534 mm
toward the larger sphere. Compute the acceleration and
the value of G.

Solution:
Acceleration: x = xi + Vxi(t) + (a)(t^2)/2

0.149m = a(1s)^2/2
a = (0.149m)(2) = 0.3 m/s/s

Is this correct for acceleration?

Could I have also used newtons second law some how to solve for acceleration?
such as....
F = (m1)(m2)(G)/.149^2 = 2.98*10^-11
F = F1.... 2.98*10^-11 = (m1)(a)
a = 2.98*10^-11/m1 = 2.9*10^-7

But obviously these answers are not the same....

 

??????