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We can do this using Newtons second law of motion.
To stop the moving block we have to apply a force against its motion. Both friction and the applied force result in decrease the velocity of the block.
Using equations of motion
`V^2 = U^2+2aS`
`a = (V^2-U^2)/(2S)`
`a = (5^2-13^2)/(2xx4.5)`
`a = -16m/s^2`
Acceleration is negative because we have a deceleration.
Let us say the force is P at the opposite direction to the movement.
Using F = ma to the direction of motion
`-P-75 = 6xx(-16)`
`P = 21N`
So the required external force is 21N at the direction opposite to the movement of block.
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