PROBLEM $7\mathrm{.}103$

Cable $\backslash ($ A B C $\backslash )$ supports two loads as shown. Knowing that $\backslash ($ b$=21\backslash$mathrm$\{$ft$\}\backslash ),$ determine $($a$)$ the required magnitude of the horizontal force $\backslash (\backslash$mathbf$\{$P$\}\backslash ),($b$)$ the corresponding distance $\backslash ($ a $\backslash ).$