Simplify.

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

b) [[$ \dfrac{4y^3}{9y \cdot y^2} = $]]

c) [[$ \dfrac{2x^6 \cdot 5x^2}{4x \cdot x^7} = $]]

d) [[$ \dfrac{x^4 \cdot y}{2x^2 \cdot 3x^2} = $]]