Nour drove from the Dead Sea up to Amman, and her altitude increased at a constant rate. When she began driving, her altitude was $400$ meters below sea level. When she arrived in Amman $2$ hours later, her altitude was $1000$ meters above sea level. Let $A(t)$ denote Nour's altitude relative to sea level $A$ (measured in meters) as a function of time $t$ (measured in hours). **Write the function's formula.** $A(t)=$ [[☃ expression 1]]