Mr. Mole left his burrow that lies below the ground and started digging his way at a constant rate
deeper into the ground. After 5 minutes of descending at a rate of 1.8 meters per minute, he
was 13.5 meters below the ground.
Let y represent Mr. Mole's altitude (in meters) relative to the ground after z minutes.
Complete the equation for the relationship between the altitude and number of minutes.