$$600$ per year for $10$ years at $12\%.$

$$$

$$300$ per year for $5$ years at $6\%.$

$$$

$$600$ per year for $5$ years at $0\%.$

$$$

Now rework parts a$,$ b$,$ and c assuming that payments are made at the beginning of each year; that is$,$ they are annuities due.

Future value of $$600$ per year for $10$ years at $12\%$: $$$

Future value of $$300$ per year for $5$ years at $6\%$: $$$

Future value of $$600$ per year for $5$ years at $0\%$