calculus
posted by Becky .
Hi i need help in this. Some of the parts i got the answer but some i don't. please help!
A ship uses $(7v^2) of fuel per hour when travelling at a constant speed of v km/h. Other expenses in operating the ship (i.e. crew and equipment) amount to $345 per hour. The ship makes a journey of 100 km.
1. Give an expression for how long the journey takes in hours.
Answer i got: Time = 100/ v
2. Give an expression for the total cost of the journey.
Answer i got: 700v^2 + 3450/ v
3. Let C(v) be the function representing the cost of the journey when the speed is v km/h. What is the derivative of C(v) with respect to the speed v?
Answer i got: 700v^2  3450/ v^2
This answer i differentiate by using quotient rule.
BELOW ARE THE 2 PARTS I CAN'T GET. PLS HELP!!!
4. What is the second derivative of C(v) with respect to the speed v?
Answer: ???
5. Complete the following statement:
To minimize the total cost of the journey the ship should travel at a speed of km/h to 3 significant figures.
Answer: ???
Appreciated!!! thank you!!

The 5 question is asking how many km/h...appreciated if anyone cld help...thank you!!