First Digit = x
Second Digit = y 
x+y=14
x=2y-4  (This comes from the relationship explained.)
Substitute the value of x, in terms of y, from the second equation into the first.
2y-4+y=14 ---> 3y=18 (combined y's, added 4 to both sides.)---> y = 6 (divided both sides by 3.)
x+6=14  (subtract 6 from both sides.)  x=8