Question 21895
The problem tells you how the two cultures ages are related NOW:
    1st culture is x+18 days;   2nd culture is x days

It also tells you to consider their ages 10 days from NOW:
    1st culture will be  x+18+10;     2nd culture will be x+10

Then it tells you the 1st culture is twice as old as the 2nd 10 days
    from NOW.  This gives you the equation you need.

Equation:   1st = 2 (2nd)          
           x+28 = 2(x+10)

Solve for "x" and "x+8"

Cheers,
Stan H.