[On a graph, the horizontal x axis is marked from 0 to 12. The vertical y axis is marked from 0 to 10. A horizontal red line runs from point A (5, 3) to point B (9, 3). A vertical red line runs from point B to point C (9, 9), forming a reversed L-shape. The narrator reads out the onscreen text.]

NARRATOR: Question three. Three of the vertices of a rectangle are A (5, 3), B (9, 3) and C (9, 9). Find the coordinates of the fourth vertex. First, plot the three points and then complete the rectangle.

[Dotted lines are drawn vertically from point A to (5, 9) and then horizontally to point C, completing the rectangle. The corner point is marked D.]

NARRATOR: Now note the coordinates of the fourth vertex.

[Point D is at x = 5 and y = 9.]

NARRATOR: Therefore the coordinates of the fourth vertex are (5, 9).