Question

What is the formula of the newton's second law

Answer 1

F=ma
Where F is the force 
m = Mass of the Body
a = acceleration of the Body

Answer 2

There are two formulas:

Force = rate of change of momentum
F = Δ p / Δ t  -- equation 1
   = ( p₂ - p₁ ) / ( t₂ - t₁ )  = (m v - m u ) / ( t₂ - t₁ )
   = m (v - u) / ( t₂ - t₁ )
F   = m a    -- equation 2

Also, conservation of momentum of a system of bodies is also derivation from Newtons second law.
         m₁ v₁ + m₂ v₂ = m₁ u₁ + m₂ u₂          ---  equation 3