#### Answer

D (0,0)
E (2a, 0)
F (a, b)

#### Work Step by Step

D is the origin, so the coordinates are (0, 0)
E is on the x-axis, therefore the coordinates are (2a, 0)
F is on the perpendicular bisector of DE so its x coordinate is a, but we don't know the height of the triangle, so we need another variable:
F (a, b).