Hi, Annabelle! Just combine the "like terms" -- in this case, combine the every term with an "s" (so...2s + s, which would give you 3s), then combine the terms with a "t" (so you would add the 3t and the 7t, giving you 10t). Your answer would be 3s + 10t.
Hope this helps!
p.s. Here's one for you to try yourself: What is the answer to 10a + 18b + a + 20b?