when two lines are perpendicular, switch the numerator and denominator of the given slope, add a negative sign in the front, you will have the slope of the other line.
if the slope is an integer, for example 3, it is the same as 3/1, the slope of the perpendicular line is then negative 1/3, (-1/3)
in this case, the slope of CD is -(-2/1)=2