Answer:
The rock climbers initial height above sea level is:
 50 meters.
Explanation:
The expression which is used to represent the elevation above sea level, in meters, of a rock climber after x minutes of climbing is given by:
 2x+50
Now, we are asked to find the rock climbers initial height above sea level i.e. we are asked to find the value of the expression when x=0.
Hence, when x=0 we have:
2x+50=2×0+50
 = 50 
Hence, the answer is: 50 meters.