A100 n force gad a soccer ball flying through the air at 20m/s^2 what is the mass of the soccer ball

We know that F=m·a .  For this particular problem, since 'm' is what we need to find, we can divide each side of the formula by 'm', and then it says m=F/a .  Much more convenient.  

Now all we have to do is write "100N" where it says "F", and write "20m/s²" where it says 'a'.  Then the formula is m=(100N/20m/s²).

When you massage that out, it finally says "m=5 kg".

But I gotta tell ya, that's no soccer ball.  On Earth, anything with 5 kg of mass weighs a hair more than 11 pounds !  You wouldn't want to play a game where you had to kick that thing or hit it with your head.  5 kg is more like a bowling ball.