D == David's age now and C = Charles's age now
8 yrs ago: (C-8) = 2(D-8) + 1 => C - 2D = -7
8 yrs from now: (C+8) = (7/6)(D+8) => 6C - 7D = 8
where I have cleared fractions and collected like terms in the equations
If you don't know how to clear fractions and collect terms, please message me and I will clarify
From the first equation C = 2D-7; substitute in the 2nd equation
6(2D-7) - 7D = 8
12D - 42 -7D = 8 => 5D = 50
D=10...also C = 13
