If friction causes a cart to speed up, when there is no longer an Fa, why does friction also slow it down? and how?

The cart is actually undergoing a "collision" with the earth.

Nothing can speed up or slow down without a force.

Friction between the tyres and the earth provide the force needed to accelerate the cart. Energy is still required and that comes from elsewhere.

Friction between the tyres and the earth provide a force needed to slow the cart. Without this the cart would skid uncontrollably and brakes cannot work.

There are other frictions that work the entire time.

Friction between the air and the cart always slows it down.

Friction between the axles and the bearings always slows it down.

There are many different places where friction is involved.

But the only friction that USES energy is one where two surfaces move relative to each other.

To a large extent this is not true about the tyres on the road. They roll but do not slide.

So this friction is not consuming much energy ( the only movement is due to the flex of the tyres. There is no movement between the tyre and the road ).

OK. Now all these OTHER frictions use energy. Unless the engine is adding energy then it must come from the loss of kinetic energy. ie the cart slows down unless it is powered.

Yet the friction between the tyres and the road actually pushes the car forward the entire time that the vehicle is being powered.

Because there is still friction on the wheels and what they are connected to that is turning as well as friction against the air its moving against.

Friction cannot cause anything to speed up. Tell us how you got this strange idea.