Answer: 421.264 grams.
Explanation: This problem is essentially testing you on your knowledge of percent mass except with one extra step.
We are given that there are 2kg or 2000 grams of ore. This ore is %15.1 Antimony. We can multiply 2000 grams by .151 to find the amount of Antimony which is 302 grams. We don't want the amount of Antimony though, we want the amount of Stibnite. To find the amount of Stibite, we can multiply the amount of Antimony by 1/(the percent mass of Antimony in 1 molecule of Stibnite. To find the percent mass we divided 2(molar mass of Sb) by the molar mass of one molecule of Stibnite. This calculation is 2(121.67)/339.69. This is the percent mass of Antimony in Stibnite. We take the inverse because we want to find out how much Stibite there is per amount of Antimony. We do 1/(((2(121.67))/339.69) and multiply that by 302 to find the total amount of Sb2S3.
Hope this helps!