Why is newton's first law of motion sometimes called the lae of inertia?

Newton's 1st law of motion states that an object at rest stays at rest and an object in uniform motion along a straight line stays in motion unless acted upon by an unbalanced force. 
This law talks about the tendency of an object to resist any change in its state of rest or uniform motion, this is called Inertia. Thus Newton's 1st law of motion is also called the Law of Inertia

Velocity=total displacement/timefrom house to store is 500 from store to neighbor is 200 therefore total displacement=500+200=700metrestherefore,700metres/910seconds=0.8metres per second

The relationship of both of them