You divide $4.20 by a dozen (12).
This gives you $0.35 per donut
For the next one, you divide $7 by 4 to get the individual price. This gives you $1.75 per cookie, but you still need to multiply it by 9 to get the total price of 9 cookies.
When you multiply 9 by 1.75, your final answer for the second one is $15.75.
Hope this helps :)