We now have two equations that together make up a mathematical model of motion that includes position, velocity, acceleration, and time. Equation (4.2) describes velocity and equation (4.3) describes position.
Using the equations to find distance
Some problems require both equations. For example, consider a car that starts from rest with a constant acceleration of 5 m/s2. How far does the car go before it reaches a speed of 30 m/s (67 mph)? This problem asks for a distance but it gives you speed and acceleration. There are two unknown values: time and position. Therefore, to solve the problem you need both equations.
Equation (4.2) allows you to solve for the time in terms of the known final velocity and acceleration.