Answer: y = (3^(x-1))*1500 
 
Step-by-step explanation: 
 
We want the relationship 
 
y = f(x) 
 
where y is the number of songs, and x is the number of years. 
 
We know that in the first year, x = 1, the site has 1,500 songs. 
 
The next year, x = 2, the number of songs is tripled, then we will have: 
 
1,500*3 = 4500 songs. 
 
The next year, x = 3, we will have: 3*(3*1500) = (3^2)*1500. 
 
when x = 4, the number is tripled again: 
 
3*(3^2)*1500 = (3^3)*1500 
 
We already can see the pattern, for the year x, the site will have: 
 
y = (3^(x-1))*1500 songs. 
 
This is the equation we are looking for.