Explanation

(b) $\overrightarrow{S} = \left( 10\widehat{i} - 2\widehat{j} + 7\widehat{k} \right) - \left( 6\widehat{i} + 5\widehat{j} - 3\widehat{k} \right) = 4\widehat{i} - 7\widehat{j} + 10\widehat{k}$
$\overrightarrow{W} = \overrightarrow{F} \bullet \overrightarrow{S}$
=$\left( 10\widehat{i} - 3\widehat{j} + 6\widehat{k} \right) \bullet \left( 4\widehat{i} - 7\widehat{j} + 10\widehat{k} \right)$
$= (40 + 21 + 60)$J=121 J