To bring something to a stop the same force that was applied to speed it up can be used to stop it. If a greater force is used it will stop quicker.
Trust me, i'm a k12 student and its motor
Given that force is applied at an angle of 30 degree below the horizontal
So let say force applied if F
now its two components are given as


Now the normal force on the block is given as



now the friction force on the cart is given as



now if cart moves with constant speed then net force on cart must be zero
so now we have




so the force must be 199.2 N