(I'm learning this right now!)
y = 3x - 3
If a line is perpendicular to another line, they have negative reciprocal slopes.
So you would use the slope -1/3.
Use y - y₁ = m(x - x₁) to get an equation in slope intercept form.
Plug the numbers in.
y - 1 = -1/3(x - 3)
y - 1 = -1/3x + 1
y = -1/3x + 2
An equation of a line perpendicular to line CD is y = -1/3x + 2.