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.