Hey it isnt letting me submit my answer to your question on the Japanese chart, so imma just submit it here
Answer:
Your answer should be 15.68 grams.
Explanation:
Seeing as 1 mole has a mass of 56 g, 56*0.28 would get you 15.68 g.
It would be 4.2, hope this helps.
0.250 L*3M=0.250 L*3mol/L= 0.750 mol