I am trying to isolate s' from the equation but I forgot my algebra. So how do I get from step 2 to the answer provided in step 3?

Step 1 n_a/S ₊ n_b/S' = 0 (This is the equation )

Step 2 n_b/S' = - n_a/S (standard move from one side to the other)

Step 3 S' = -n_b/n_a S (This is the rearranged formula isolating for S', how did it evolve from step 2 to step 3)