Answer
$-\frac{b^2}{2m}$
Work Step by Step
The height of the triangle is b. Thus, we must find the base. Since the line starts at b and goes down at a rate of m, it will intersect the x-axis at $x=-\frac{b}{m}$. Thus, the area is: $A = (1/2)(base)(h) = b(-\frac{b}{m})(1/2) = -\frac{b^2}{2m}$
