A small sphere with mass 5.00.*10-7kg and charge + 3.00μC is released from rest a distance of 0.400 m above a large horizontal insulating sheet of charge that has uniform surface charge density σ=+8.00pC/m2.

I will assume you are asking what the initial acceleration of the sphere is since the information provided seems to indicate that.

First we need to know Newton's Law

F=ma.

We know the mass of the sphere and we want a so we solve to get

a=F/m.

Now we need the force on the charged sphere. This is given by the electric field, E and the charge, Q. The relationship is F=Q*E. (Recall that the electric field units can be expressed in Newtons/Coulomb).

Now the electric field above a large (~infinite) sheet of charge with a known charge density σ, is given by

E = σ / (2ε0)

Plug in your values of σ, to get E, then the sphere charge Q to get F, the the mass into a = F/m to get the acceleration