Here what we need to recognise is the first law of Newton.
His first law of motion says that an object will be at rest or travelling in constant velocity until an external force applied on it. so in this stage the net force on an object is zero.
The cart is travelling at a constant speed. So the net force on the cart should be zero according to the first law of Newton.
There are two forces acting on the cart. One is the horizontal force that pulls the cart and the other is the friction acting against the motion. Since the net force is zero the force that pulls the cart should be equal to the friction force.
So the force that pulls the cart is 10N.