Let's convert the quantities in SI units first.
The force exerted by the horse is
[tex]F=200 lb=889.6 N[/tex]
and the distance covered is
[tex]d=3 mi=4828 m[/tex]
Therefore, the work done by the horse is equal to the product between the force applied and the distance:
[tex]W=Fd=(889.6 N)(4828 m)=4.29 \cdot 10^6 J[/tex]
