we are given that
a positive number represents above sea level and a negative number represents below sea level
(A)
(i)
15 m above sea level means positive number
so, that number would be +15m
(ii)
10 m below sea level means negative number
so, that number would be -10m
(B)
"-21m"
Since, sign is negative
so, it will be below sea level
so, 21m below sea level.................Answer