I'm so sorry if this is wrong but it might be 24.
Answer:
c
Step-by-step explanation:
what you do is you find how many sides there are witch is 6 and multiply that by 6 witch would equal 36 now all you do is multiply 3x36 witch equals 108cm square
I believe #2 would be the best answer. Either that or #1.
Since 1sand=3.5compost you can set up the ratio 1/3.5 or 1 sand for every 3.5 compost. If you change the number of sand to three, then you multiply the top by three since 1*3=3. To keep the mixed number equal you would have to multiply the bottom number by three as well. This means that you can multiple all the sand by 3.5 to get how much compost is needed.