Question

The equation of line CD is (y−3) = − 2 (x − 4). What is the slope of a line perpendicular to line CD?

Answer 1

parallel lines have the same slope, perpendicular lines have the negative reciprocal slope of each other, like this:
\[\large{m_1=-\frac{1}{m_2}}\]

Answer 2

first, you need the slope of that line, then find it's negative reciprocal