$($a$)$ 'not' and $\text{'}^(^())\text{'}$ form a functionally complete collection of logical operators because we can express AND, OR$,$ and IMPLICATION using these operators. $($b$)($i$)$ No contractors are dependable:

$.($ii$)$ Some contractors are engineers:

$.($iii$)$ Some engineers are not dependable:

$.($iii$)$ follows from $($i$)$ and $($ii$)$ because if there exists a contractor who is an engineer, then by $($i$),$ this contractor is not dependable.