You need to find the unit rate of 3 : 1800 first by dividing 1800 and 3
1800 ÷ 3 = 600
So this means for every 1 pound bag it covers 600 square feet. You now need to multiply 600 and 4 to see how many square feet 4 pounds cover.
600 × 4 = 2400
So each 4 pounds covers 2400 sq ft. Now you have to divide 8400 and 2400 to find how many bags of 4 pounds you need
8400 ÷ 2400= 3.5
You need 3.5 bags of 4 pounds.
Technically you would need 4 bags because you’d have to round it up.
Hope this helps you!! :)