Writing linear functions word problems

Mr. Mole left his burrow that lies below the ground and started digging his way deeper into the ground, descending at a rate of $1.8$ meters per minute. After $5$ minutes, he was $13.5$ meters below the ground. Let $A(t)$ denote Mr. Mole's altitude relative to the ground $A$ (measured in meters) as a function of time $t$ (measured in minutes). **Write the function's formula.** $A(t)=$ [[☃ expression 1]]