Write the following statements as formulae in propositional calculus (using propositional variables and connectives). Clearly indicate the meaning of the variables!

a) $x$ is a natural number and $y$ is a rational number.
b) $x$ is not a natural number or $y$ is not a rational number.
c) If $p$ is a prime number and $p>2$ then $p$ is odd.
d) if $x^2-x-6=0$ then $x=-2$ or $x=3$.
e) if $x=-2$ or $x=3$ then $x^2-x-6=0$.
f) if $x=\not-2$ and $x=\not3$ then $x^2-x-6=\not0$