Okay, my doubt is question says "7 of the counselors have sailing certificate", it didn't say "7 of the counselors have sailing certificates only", then why do we have to assume that in 7 counselors, they don't have the other certificate?

7 of the counselors have sailing certificates only.....If this would have been the scenario we would not be subtracting the number of people having both the certificates from them.
n(A U B) = n(A) + n(B) - n(A ∩ B)
in the above formula "n(A)" is all the number of counselors having sailing certificate------.to make it "number of counselors having only sailing certificate" we are subtracting n(A ∩ B) from it.

hope it helps
I am not good at explaining.

I understand now, I made a mistake while making Venn diagram.

The first sentence of the problem means that 4 counselors have neither sailing certification nor first aid certification. Therefore, the second sentence means 2 counselors have both, and 7 have sailing certification.

We can use the formula for overlapping sets and substitute the known values to solve for x, the number of counselors with a first aid certification:

Total = Sailing + First Aid - Both + Neither

22 = 7 + x - 2 + 4

22 = 9 + x

13 = x

Doesn't it say members that have both certificates are 4?
'All but 4 have ....'

EDIT: Okay, sorry I misread the question. The answer is 13.