Newton's second law of motion says that the acceleration $\vecphy{a}$ of an object is inversely proportional to its mass $m$ and directly proportional to the force $\vecphy{F}$ acting on it: $$\vecphy{F}=m\vecphy{a}$$
Newton's second law implies Newton's first law. Newton's second law says that if the force is zero, the acceleration is also zero. We know that if the acceleration is zero, the velocity of the object is constant:$$\vecphy{F}=0 \Rightarrow \vecphy{a}=0 \Rightarrow \vecphy{v}=\text{constant}$$