Question 34553
Let a-b=x then your problem becomes:
x^2-3x-18=0
(x-6)(x+3)=0
x=6 or x=-3
Therefore a-b=6 or a-b=0

2nd question:
What is a-b-14 if a-b+3=0
If a-b+2=0
Then a-b==3
Substituting that into a-b-14
you get -3-14=17
So a-b-15=17 if a-b+3=0
Cheers,
Stan H.