Physics ( Newton's law)

The resistance is friction if only Newton's law is concerned.

If you just consider about the Newton's Law, you have to first assume that the frictional force acting on the ball by the grassland is constant , then we can use the formula :

v^2 = u^2 + 2as

For your situation , because the ball finally stopped , v = 0 and s = 20
And for a , Friction = ma , a = friction / 1.2

After calculation , you can easily find out that the magnitude of friction = 1.2u^2 / 40 (N).


***
In addition , or just for your further reference , the resistance also include the air resistance other than friction.

For this , the magnitude of the Air resistance( Drag force) = 1/2 C (rho) A v^2 ,

C is the Drag coefficient with a value between 0.4 and 1 .

(rho) is actually a greek word , its physical meaning is the air density(1.21kg/m^3)

A is the cross-section Area of the object

v is the velocity.

I hope that my explaination would not be too long and it is useful for you.

since v^2=u^2=2as,
when v=0, s=20,
a = -u/40
by Newton's second law,
F=ma,
resistance= 1.2*(u/40) =3u/100