500% of what number is 3,450

What is the missing step in this proof ?

David is spreading mulch on a triangular area of his flower bed. the coordinates of the vertices of the area are (1, 3), (9, 3), and (4, 6). what is the area of the triangle if each square has an area of 3 square feet?

Binh says the point of intersections is (0,-3) which statements identify the errors binh made , check all that apply