#### Answer

2 ft

#### Work Step by Step

Let the specialist needs two pieces of duct 3$\frac{3}{4}$ ft and 2$\frac{1}{4}$ ft in length.
the excess that will be left after the 3$\frac{3}{4}$ ft length iscut off from the 4 ft long piece = 4 - 3$\frac{3}{4}$ ft
= 4 - $\frac{15}{4}$ ft
= 4 $\times$ $\frac{4}{4}$ - $\frac{15}{4}$ ft
=$\frac{16}{4}$ - $\frac{15}{4}$ ft
= $\frac{1}{4}$ ft
the excess that will be left after the 2$\frac{1}{4}$ ft length iscut off from the 4 ft long piece = 4 - 2$\frac{1}{4}$ ft
= 4 -$\frac{9}{4}$ft
= 4 $\times$ $\frac{4}{4}$ - $\frac{9}{4}$ft ft
=$\frac{16}{4}$ - $\frac{9}{4}$ft ft
= $\frac{7}{4}$ ft
Total excess left = $\frac{1}{4}$ + $\frac{7}{4}$
= $\frac{8}{4}$
2 ft