a) $(-\dfrac{28}{9}).(-\dfrac{38}{14})$
$=\dfrac{28.38}{9.14}$
$=\dfrac{2.38}{9}=\dfrac{76}{9}$
b) $(-\dfrac{21}{6}.(-\dfrac{24}{7})$
$=\dfrac{21.24}{6.7}$
$=\dfrac{3.4}{1.1}=12$
c) $|-\dfrac{12}{17}|.(-\dfrac{34}{9})$
$=\dfrac{12}{7}.(-\dfrac{34}{9})$
$=-\dfrac{12.34}{17.9}$
$=-\dfrac{4.2}{1.3}=-\dfrac{8}{3}$