How can one put this recurrence relation in terms of the first value of a?

Question

How does one put the recurrence relation:$$a_{2k}=\frac{(-1)^k}{4kl+2k(2k+1)}a_{2k-2}$$

Where $l$ is a non-negative integer. In terms of $a_0$ so that:$$a_{2k}=\frac{(-1)^kg(k)}{f(k)}a_0$$