Aball is thrown upward at a velocity of 20m/s. what is the velocity after 3 seconds?

This is a perfect problem to use positive and negative velocities.

Let's call the positive direction the direction that points UP. 
Then the negative direction is the one that points down.

The acceleration of gravity on Earth is 9.8 m/s² down.  That means
that any object with no forces on it except gravity gains 9.8 m/s of
speed downward every second.

OK. Ready to go to work:

The ball's initial velocity is +(up)20.0 m/s .
Every second after the toss, the ball has -(down)9.8 m/s more velocity.

The ball's velocity after 3 seconds is

            +20 + 3(-9.8)  = 

              20  -  29.4  =    - 9.4 m/s .

That means it's velocity at that time is  9.4 m/s pointing down.

Gas is all around us which means that the atmosphere