Question

a car whose mass is 1000kg is traveling at a constant speed of 10m/s. Neglecting any friction how much force will the engine have to supply to keep going the same speed ?

Answer 1

This next statement is a big deal.  It should be up on a board, surrounded
by flashing red and yellow lights, and hung on the wall of every Science
classroom.   Although we never see it in our daily lives, it's fundamental to
the workings of the universe, and it's also Newton's first law of motion:

Without friction, it doesn't take ANY force to keep a moving object
moving.  Force is only required to change the object's speed, or to
change the direction in which it's moving.

The answer to the question is:  On a level road, and neglecting any friction,
the engine doesn't have to supply ANY force to keep the car going at the
same speed.

Answer 2

Since you already know:

Force  =  mass *  acceleration.

acceleration =  (Velocity Change) / time.

Since we were told that velocity is constant, therefore Velocity Change = ZERO.

Therefore acceleration = ZERO.

Force = 1000 kg * ZERO = ZERO.

Force = 0 N.

Cheers.