Question

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.

Answer 1

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