Ok, so let's talk this one through...if she buys 5 stamps for 2 weeks she would have 10 stamps at the end of two weeks...so what did we do? We multiplied 5*2=10. So that means that s*w is part of the equation. If she started out with 58 stamps to get the total number of stamps at the end of two weeks what would we do with the 58? We would add it to the 10 stamps we already have. So far so good? So the equation that would be needed is "sw+58" or "58+sw" which is answer "b". Hope this helps.