\(a,\dfrac{5}{7}-\dfrac{11}{4}\)
\(=\dfrac{20}{28}-\dfrac{77}{28}\)
\(=\dfrac{20-77}{28}\)
\(=-\dfrac{57}{28}\)
\(b,\dfrac{15}{7}:-\dfrac{18}{14}\)
\(=\dfrac{15}{7}\times\left(-\dfrac{14}{18}\right)\)
\(=\dfrac{15\times\left(-14\right)}{7\times18}=-\dfrac{210}{126}=-\dfrac{5}{3}\)