Given,
The dimension of the rectangular frame is 3 feet by 6 feet.
Required
The length of the diagonal brass.
The length of the diagonal is calculated as,
[tex]\begin{gathered} Diagonal=\sqrt{(length)^2+(base)^2} \\ =\sqrt{(3)^2+(6)^2} \\ =\sqrt{9+36} \\ =\sqrt{45} \\ =6.7\text{ feet} \end{gathered}[/tex]Hence, the length of the diagonal is 6.7 feet.