How to find out if a table is proportional

Evaluate the expression for the given value of the variable 7a - 4a for a =8

This is about circle theorems. someone asap : (

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