Maximums through an unknown function

Hi. I have a very long question which has 2 parts... I've worked out part 1 but part 2 is giving me problems.

__Task 1__

Imagine that you are the pilot of a light aircraft which is capable of cruising at a steady speed of **a **km/hr in still air. You have enough fuel on board to last **b **hours.

You take off from the airfield and, on the outward journey, are helped along by a **c **km/hr wind which increases your speed relative to the ground to **a + c **km/hr.

Suddenly you realise on the return journey you will be flying into the wind and will therefore slow down to **a-c **km/hr.

What is the **maximum distance** that you can travel from the airfield, and still be sure that you have enough fuel left to make a safe return journey? Results must be **verified.**

**a = 350 b = 4.5 c = 40**

I found the answer to part 1 through the formula:

t = d / s by manipulating it into:

t = d / (a+c) + d / (a-c)

I subbed in the values for a, c and t and the value for a maximum __one way__ was 777.21 km. (1554.428km there and back).

__Task 2__

**Determine **a function linking the distance from the airfield (km) and the time of the flight (hrs). Hence, **determine **the **greatest distance **the plane can travel from the airfield and what **wind speed **will allow this to occur. You must demonstrate algebraic techniques and calculus. Conclusions must be **verified. **

Task 2... well, I'm honestly not quite sure where to start. (Doh) I have derived the equation which I worked out to be:

t' = 2da^2 + 4dac + 2dc^2. That's all I've got so far... I could sub in the value of a (350), but that still leaves me with the unknowns of d, c and t.

Help would be much appreciated.

Thankyou.