In a given geometric sequence gn, g4 = 5 and g6 = 11.25. What is g14/g17?

The answer is 8/27. The book uses the geometric sequence formula... I don't understand why they plug certain numbers in. For example... The formula is an=a1r^(n-1)

How do you know what to put in for a1? The book puts in g4 as "a1."