Line EF passes through the points (0, 1) and (3,3). Line JK passes through the points (8, 2) and (6, 5). These lines are

Slope of first line = rise/run  =  y2-y1/(x2-x1)  =  (3-1)/(3-0) = 2/3

Slope of second line =  3/-2 = -3/2  

The second slope is the negative inverse of the first, so the lines are perpindicular