What is 14/42 divided by 45/15 (/ is the line that divides the numerator and the denominator because these are fractions)

To begin, yes, the line that divides the numerator and denominator is there because these are fractions. Even though 45/15 is improper, it is still a fraction. When you divide fractions, you are simply multiplying by the reciprocal:
(14/42) / (45/15) = 14/42 x 15/45
Now, simplify these fractions by finding the GCF of the numerators and denominators and dividing:
GCF of 14 and 42: 14  (Now, divide both sides by 14 to get 1/3)
GCF of 15 and 45: 15  (Now, divide both sides by 15 to get 1/3)
Now, our problem is 1/3 x 1/3
Once multiplying straight across, you'll get:
1/9       (1 x 1 = 1 / 3 x 3 = 9)
Hope this helps!