The worlds two highest dams are both in tajikistan. the rogue dam is 35 meters taller than the nurek dam. together they are 635 meters tall. find the height of the nurek dam.

This is a sum and difference problem, with the equations
R-N=35
R+N=635
The solution of which is
R=(635+35)/2=335
N=(635-35)/2=300

In general, the sum and difference problem can be solved (most of the time mentally) using
Larger number = (sum + difference)/2
Smaller number = (sum - difference)/2

It is 6% because if you subtract 69-58=6 so 6% is the answer to your question.