Please help with this question

https://preview.redd.it/fw5e7gp2sr9e1.png?width=1142&format=png&auto=webp&s=e0b071bb4be5f497e6e057ea67a141a933c74f79

So I just used n(total)=n(a)+n(b)+n(c)-n(a int b)- n(b int c) -n(c int a) +n(a int b int c)
so 12+5+7-2-3-6+x=13+x

13+x is less than or equal to 41(total number of applicants), I can't say shit about the value of x. Where am I going wrong with this approach?