jmc

We know that, for a triangle, area = 1/2(base)(height), which equals 30 in this problem. We also know that the height of the triangle is 4 if we use the horizontal leg on the x-axis as the base. Now we can plug this information into the equation to find the length of the base that runs along the x-axis. The equation is $(1/2)(b)(4)=30$, so $b=30/2=15$. Since the 3rd vertex is on the x-axis we know that it extends straight left 15 units from the vertex at (0, 0), bringing us to the point $\boxed{(-15,0)}$.