Question 433005
Let f = Father's age
Let s = Son's age
Given:
f + s = 84
In 6 years' time, the father's age will be f+6. 
Three years ago, the son's age was s-3. 
So we can write the following equation:
f + 6 = 2(s - 3)
We have 2 equations and 2 unknowns.  Solve for f in the 1st equation and put in the 2nd:
(84-s) + 6 = 2s - 6
Solve for s:
96 = 3s -> s = 32
So, the son is 32 yrs old and the father is 52 yrs old.