Recall your d = rt, distance = rate * time
now, if say, by the time they meet, Mr Cunningham has travelled "d" miles, that means Mrs Cunningham must also had travelled "d" miles as well.
However, he left 3 hours earlier, so by the time he travelled "d" miles, and took say "t" hours, for her it took 3 hour less, because she started driving 3 hours later, so, she's been on the road 3 hours less than Mr Cunningham, so by the time they meet, Mrs Cunningham has travelled then "t - 3" hours.
Well it would be (x+3)(x+3)
using the FOIL method you end up with:
x^2 + 6x + 9
:D
15 and one half is 31/2. 3 and seven eighths is 31/8. 31/2 also equals 124/8. Then the answer is 124/8 - 31/8 is 91/8. Hope that helped.