Find proportion $k$ between two arithmetic series such that $S_1(i) \ge S_2(i) k$

Question

Let's say that:

$S_1(i) = a + (i - 1) b$

$S_2(i) = c + (i - 1) d$

and I want to find the k that fulfils $S_1(i) \ge S_2(i) k$

I've tried doing the inequation $a + (i - 1) b \ge (c + (i - 1) d ) k \space$ but I don't get anywhere