First I calculated the slope of the line which goes from point A to B. This is -1/3. The line which goes from C against the side AB is has to be perpendicular against A and B. This means it has a slope which is 3.

y2-y1/x2-x1=slope

9-y1/4-x1= 3

From here I do not know how to continue. Can you please help me? Thank you in advance.