Anyway, could you help me with my computations?

First off I plan to record how long it takes to hit the ground to calculate vertical air resistance, let's say it takes 8 seconds and it starts off going 4m/s upwards (which is negative for this instance) :

Substitute:

Simplify:

Simplify:

Add 30 to both sides:

divide both sides by 25/2:

Then:

(keep in mind I made the beginning numbers up)

Thus the average acceleration of air resistance is:

Now to find the air resistance horizontally, the ball went 20m we'll say and was going 8m/s to begin with...

Start off:

Solve for a:

Substitute:

then:

Therefore:

Thus:

Finally the acceleration due to air resistance (for the horizontal movement) is:

Is that all correct?