Find the slope of both lines to conclude the relaion between the lines.
Slope of line bc = rise / run = [2-(-5)]/[3-4] = -7
Slope of line de = rise / run = [1-0] / [9-2] = 1/7
When the slopes of two lines, say m1 and m2 satifiies that m1xm2=-1, the lines are perendicular.
In this case, (-7) x (1/7) = -1, the you conclude that line bc and line de are perpendicular.