How do I do these? would you give me a start to the problem ​

How do i do this using elimination 2x+3y=12 x-y=6

Write numerical coefficient of y² in the expression 2x² - 15xy – 7y²

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?