The total work done by the pulling force is the sum of the work done against resistance and the gain in potential energy:

\(\text{Total work done} = 900 \text{ J} + 2100 \text{ J} = 3000 \text{ J}\)

The work done by the force \(F\) is given by:

\(\text{Work done} = F \times 100 \times \cos(15^{\circ})\)

Equating the two expressions for work done:

\(3000 = F \times 100 \times \cos(15^{\circ})\)

Solving for \(F\):

\(F = \frac{3000}{100 \times \cos(15^{\circ})}\)

Calculating \(F\):

\(F \approx 31.1 \text{ N}\)