John is 13 years less than 3 times as old as his son. Six years ago, he was 14 years more than twice as old as his son was then. How old is John?

Let John's age now be J and his son's current age be S.
The looking at the ages now, J + 13 = 3S : Thus S = (J + 13) ÷ 3
And looking at 6 years ago, (J - 6) - 14 = 2(S - 6) : J - 20 = 2S - 12 : 2S = J - 8 : S = (J - 8) ÷ 2
Putting the two equations for S equal to one another gives :-
(J + 13) ÷ 3 = (J - 8) ÷ 2 : 2(J + 13) = 3(J - 8) : 2J + 26 = 3J - 24 : J = 50

John is 50 years old.