What number must be added to the expression below to complete the square. x^2-18x

Hi my sisters and brothers can tell me the answer because i don’t understand the question

Galen sold tickets of his church’s carnival for a total of $2,820. children’s tickets cost $3 each and adult tickets cost $5 each. the number of children’s tickets sold was 30 more than 3 times the number of adult tickets slod. how many children’s ticket and how many adult tickets did he sell?

Factor the expression using the gcf. the expression 21m−49n factored using the gcf