S = n(a1 + an)/2
2S = n(a1 + an)
2S = na1 + nan
nan = 2S - na1
an = (2S - n a1)/n
Step-by-step explanation:

Answer:
the answer is 
Step-by-step explanation:
Try using Symbolab, I use it all the time it gives the correct answer and it gives good explanations.
hope this helps :)
It would be 21 because 7 times 3 equals 21 and the way you get that is 3 drawings per paper