a) 5.00 x 10^8 J
The work done to move the sailboat is calculated through the equation;
W = F x d
where F is force and d is the distance.
Substituting the known values from the given above,
W = (5.00 x 10⁴ N)(10 km)(1000 m/ 1km)
= 5.00 x 10⁸ J
Thus, the work done is 5.00 x 10⁸Joules