Simplify.

a) [[$ \dfrac{3}{7}a^2 \cdot 1\dfrac{1}{6}a =$]]

b) [[$ -\dfrac{7}{8}b \cdot \left(-\dfrac{2}{21}ab^3\right) =$]]

c) [[$ 3\dfrac{2}{5}c^2 \cdot 1\dfrac{5}{17}c^2 =$]]