Ok, each sandwich needs 3/8 pounds of roast beef. We also know that Marisa started with 5 pounds and now she has 2 3/4 (or 2.75 pounds).
First we need to see how much roast beef she used, this is the difference between the amount that she had at the start and the amount that she had at te end.
5 - 2.75 = 2.25 pounds
So she used 2.25 pounds.
And we also know that each sandwich needs 3/8, then we need to divide the amount she used for the amount needed for one sandwich, and the result is the amount of sandwiches created.
(2.25)/(3/8) = 6
So she 6 sandwiches were created.