Step 1: Understand the geometry and given parameters. The derrick crane has the following members and properties: Load $F = 30 \text{ kN}$ acting vertically downwards at C. Support $AB = 4 \text{ m}$. AB is vertical. Struts $EB = DB = 4.6 \text{ m}$. E, B, D, G are in the horizontal plane. Point B is supported on horizontal rollers, meaning the reaction at B is purely vertical. Crane arm BC forms an angle of $60^\circ$ with the horizontal line BG. This means $\angle CBG = 60^\circ$. Angle $ABC = 120^\circ$.