6x+11=21 i dont know what x is! Help me plz

This is so hard and it’s for a test

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?

Can someone me do math because i am having a breakdown rn because i don’t get it