Solve for x:

4x + 12 = 8 (given)

4x = -4 (subtraction)

x = -1

what is the reason for the last step in the argument?

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