We can then write an equation representing this problem as:
e−1.5mi=5.25mi
Now, add 1.5mi to each side of the equation to solve for e while keeping the equation balanced:
e−1.5mi+1.5mi=5.25mi+1.5mi
e−0=6.75mi
e=6.75mi
The plane's starting elevation was 6.75 miles
Hope this helps!