Find the limit (if it exists) of each of the following sequences.
a) xn = (2 + 3n - 4n2)/(1 - 2n + 3n2)
b) xn = (n3 + n - 2)/(2n3 + n - 2)
c) xn = √3n + 2 - √n
d) xn = (√4n + 1 - √n - 1)/(√9n + 1 - √n + 2)