a) $\dfrac{1}{7}.(-\dfrac{3}{8})-\dfrac{13}{8}.\dfrac{1}{7}$
$=\dfrac{1}{7}.(-\dfrac{3}{8}-\dfrac{13}{8})$
$=\dfrac{1}{7}.(-2)$
$=-\dfrac{2}{7}$
b) $\dfrac{1}{2}.\sqrt{64}-\sqrt{\dfrac{4}{25}}+1^{2019}$
$=\dfrac{1}{2}.8-\dfrac{1}{5}+1$
$=4-\dfrac{1}{5}+1$
$=5-\dfrac{1}{5}$
$=\dfrac{25}{5}-\dfrac{1}{5}=\dfrac{24}{5}$