The average of a set of 10 numbers is 6. nine of the numbers are as follows: 3,4,5,6,7,10,1,3,12. what's the tenth number?

How do i do this? i don’t understand

Icould really use some with this question.

Afarmer has a large field that is x feet in length. he wants to fence in a rectangular section in the middle of the field , leaving a length of 100 feet of open field behind each end of the fenced rectangle.he also wants the width of the fenced-in space to be 100 feet less than its length. find the expressions to represent the length and width of the fenced section of the field