Melissa and brian are at the base of a moutain. melissa hikes to a location 27 meters above sea. level brian hikes to a location 21 meters below sea level

The difference is 48 meters

Step-by-step explanation:

The question is

What is the difference of the hikers' altitudes?

Let

x -----> the Melissa's altitude in meters

y ----> Brian's altitude in meters

we have that

---> is positive because is above sea level

---> is negative because is below sea level

Remember that the sea level is the altitude equal to zero

Find the difference

To find out the difference of the hikers' altitudes, subtract Brian's altitude from Melissa's altitude

so

Remember that (-1)(-1)=+1