Simplify 150-(0.4 x 150 + 15)

How do you use the elimination method for this question? explain, because i really want to understand!

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

Common binomial factor of 6x(3x+7)+5(3x+7)