A $1\mathrm{.}22\mathrm{.}4-$m sheet of plywood is temporarily held by nails at $D$ and

$E$ and by two wooden braces nailed at $A,B$ and $C.$ Wind is blowing

on the hidden face of the plywood sheet, and it is assumed that its

effect may be represented by a force $Pk$ applied at the center of the

sheet. Knowing that each brace becomes unsafe with respect to buck$-$

ling when subjected to a $1\mathrm{.}8-kN$ axial force, determine $(a)$ the maxi$-$

mum allowable value of the magnitude of $P$ of the wind force,

$($b$)$ the corresponding value of the $z$ component of the reaction at $E.$

Assume that the nails are loose and do not exert any couple.