When applying a horizontal force of 30n, an object of mass 6 kg accelerates at 4m/s2. the force of friction on the surface must be n

Using Newton's Second Law, F = ma, where F is the net force

So the net force is:

F = (6kg)(4m/s^2) = 24N

Since you are applying a horizontal force of 30N, we can find the force of friction by the difference of the net force and the applied force.

30N-24N = 6N