To find the single additional force that will produce equilibrium, we need to resolve the given forces into their components and find the resultant force. The additional force will be equal in magnitude and opposite in direction to this resultant force.

Resolve the forces into horizontal (x-axis) and vertical (y-axis) components:

Horizontal component, \(X = 18 + 12\sin 60^\circ - 8\sin 30^\circ = 14 + 6\sqrt{3}\).

Vertical component, \(Y = 8\cos 30^\circ + 12\cos 60^\circ = 6 + 4\sqrt{3}\).

The magnitude of the resultant force is given by:

\(\sqrt{(14 + 6\sqrt{3})^2 + (6 + 4\sqrt{3})^2} = 27.6 \text{ N}\)

The direction of the resultant force is given by:

\(\arctan \left( \frac{6 + 4\sqrt{3}}{14 + 6\sqrt{3}} \right) = 27.9^\circ\)

Thus, the additional force required for equilibrium has a magnitude of 27.6 N and is directed 27.9° below the negative x-axis.