Consider the following premises. w or ~r_2. w -> t. s -> r_2. Can we infer s -> t from them?
 From w or ~r_2 we can infer r_2 -> w. From r_2 -> w, w -> t we can infer r_2 -> t. Finally, from s -> r_2, r_2 -> t we can infer s -> t. Therefore, the answer is yes.