Why is the Newton's second law called the equation law?
This is because it gives us an equation relating force to acceleration. It is so useful that, it is used to furnish a definition of force.
Implies, F=M.dv/dt (Since, p = Mv, the momentum)
This gives, F=M.a considering the familiar result a = dv/dt
We may also note that, when F = 0, a = 0 , thus it elegantly account's for Newton's first law.
Similarly, it maybe showed using vector algebra that it reproduces the third law as well.