How much force is needed to stop a 80 kg football player if he decelerates at 5m/s?

Use Newton's 2nd law of motion.
Here it is:

                     Force  =  (mass)  x  (acceleration)

                                 = (80 kg) x (-5 m/s²)

                                 =      -400 Newtons .

I called the acceleration negative because the player is slowing down.

The force comes out negative because it's in the direction opposite to
his motion.