determine whether the side

through the points (2, 3) and (11, 6) is perpendicular to the side through the points
(2, 3) and (_3, 18).

Calculate the slopes of each of the lines between the pair of points. Hint: The slope of the line that is the first side is (y2-y1)/(x2-x1)

=(6-3)/(11-2) = 1/3
If the product of the two slopes is -1, they are perpendicular. If not, they aren't.