Answer:
see below
Explanation:
It's a matter of carefully reading the problem statement, formulating expressions that match the words, then rearranging those to matrix form.
Useful initial expressions might be ...
x + y + z = 2100 . . . . . . the total elevation gain is 2100 ft
x = 2z -100 . . . . . . . . . . the first leg is 100 ft less than twice the last
y = (x +z)/2 . . . . . . . . . . the middle leg is the mean of the other two
__
The second of these equations can be rewritten as ...
-x + 2z = 100
The last of these equation can be rewritten as ...
0.5x - y + 0.5z = 0
These forms together with the first of the equations above can be written as the augmented matrix below.