I assume you mean an isosceles right triangle since only right triangles have a hypotenuse. Then the two legs are perpendicular. This means that their slopes are negative reciprocals of each other. So the product of their slopes is -1. Therefore the product of the three slopes is (-1)a = -a.
Extra detail: If one of the legs has slope b, then the other leg has slope -1/b. So the product of the three slopes is b(-1/b)(a) = -a.
Technical note: This is not really a good problem. There is actually the possibility that the product is undefined. If the legs are parallel to the coordinate axes, then the vertical segment has an undefined slope. So clearly this is not an actual SAT question.