[Help Me!] Linear motion equations with drag resistance

I've been taught about the normal linear motion equations now i've been set this task

- Identify the phenomenon that causes objects to hit the ground at different times [Completed]
- Determine the value of engineering coefficients
- If possible attempt to develop a mathematical model that takes both gravity and this phenomenon into account to accurately predict the fall time for different spherical objects.

Other information

Acceleration: 9.81ms^-2

Fall height of object(s): 6M

Shape of object(s): Sphere

What I've done

For part 1: I deduced the phenomenon is air resistance/drag.

For part 2: Researched about drag and found out that

Where F_d: Force of drag, p=density of fluid(1.1877 @298K/25C),

v=speed of object

a=reference areaπ(pi*D*^2/4 for a sphere),

C_d=drag coefficient(0.47 for a smooth sphere),

v^=is the unit vector indicating the direction of the velocity (the negative sign indicating the drag is opposite to that of velocity).

For part 3

Not sure for this but was thinking of using v=u+at rearranging for time, draw force diagram calculate the modulus of the force then put in values