List formulas for: area of a triangle

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

Grant simplified the expression 1.5(-3.2 + 2.5) his work is shown below explain the error in grants work

The focus of a parabola is (-4,-5) and its directrix is y=-1 find the equation