The red is an oxygen molecule. 1 oxygen and 2 hydrogen =h2o or water!
The molar mass or As is 74.9g. Start with 281g multiply by (1mol/74.9g) = 3.75 mols
Put what you don’t want on the bottom (in this case grams) so they the u it’s cancel (if you multiply by grams and the divide by grams they cancel. Ultimately leave what you want on the top ( This case mols) then do the math