You know the acceleration of a pushed cart and you want to find the net force, What other information do you need to find the force?
In order to find net force with acceleration, you need to have mass of the object. Net force is calculated by multiplying mass and acceleration. Learn more from http://easycalculation.com/physics/classical-physics/newtons-second-law.php.
The carts mass. Since Force = Mass times Acceleration.
The force and mass are needed if you apply it to a mass. The formula is A=F/M so if you push a cart weighing 10kg with 20N of force, it has an acceleration of 2(I forget the units, sorry)
You have F = M*A, you need two of those.