.
Having problem like this, first step is to check if the data is consistent.
So, we check how many people are engaged/involved in the described activities.
For it, we calculate the number of those, who are involved in any of singing or dancing.
Their number is 18 + 19 - 7 = 30.
It means that of the group of 35 people, 5 perform some other activities other than singing or dancing.
Now I can answer the problem's questions.
(a) To find how many sang only, subtract from singing those who involved in both singing and dancing
18 - 7 = 11 people sang only. ANSWER
(b) To find how many danced only, subtract from dancing those who involved in both singing and dancing
19 - 7 = 12 people danced only. ANSWER
(c) The number of those who neither sang or danced is 35 - 30 = 5. ANSWER
Solved anf thoroughly explained.
All question are answered.