Answer: 18 square units
Work Shown
x = horizontal leg = base
12-x = vertical leg = height
area = 0.5*base*height
area = 0.5*x*(12-x)
area = -0.5x^2 + 6x
Use a graphing calculator to find that (6,18) is the highest point on this parabola. The parabola opens downward because the leading coefficient -0.5 is negative.
Another way to find the highest point is to find the roots of y = -0.5x^2+6x, and then find the midpoint of the roots. This will determine the axis of symmetry. Plug this value into y = -0.5x^2+6x so you can find the y value.
(6,18) being the highest point means x = 6 leads to maxArea = 18.
If x = 6 is the base, then 12-x = 12-6 = 6 is the height. This right triangle is isosceles.