Question concerning two bodies in space

Two indentical spheres with radius 15cm and mass 100kg are 6mm apart from echother in space. My problem is to find how long it will take before gravitational attraction will cause the two spheres to come together?

What I have found out so far is:
I have the gravitational force Fg= 7,12*10^-6 N,
and used newtons 2. law to find the spheres acceleration a=7,12*10^-8 m/s^2

I know the acceleration is not constant, because the gravitational forces increase as the spheres move towards eachother. But I think I will neglect this, and treat it as constant.

If I do so, could I use the equation:
X = x0 + v0x +1/2*ax*t^2
t = sqrt((2*x)/ax)= 84269sek ,
x = 0,003m, v0x = 0 m/s, x0= 0 m

I don't know if I can treat v0x as being 0, should it be v0x = a?

Is this way of, or am I on the right track?

all kind of help would be apprciated, thx

 

Yea I know about calculus, but the question says you get 90%/points for a reasonable answer, and 100% for a correct. I will settle with 90%, and use
X = x0 + v0x +1/2*ax*t^2 (although I'm not sure this is the right way), and use V0x = 0.

Thanks