A line goes through point , point and point . What is the value of ​?

The slope from A to B must equal the slope from B to C.

 

(k - 1) / (19 - 9)  =  (0 - k) / (7 - 19)

        (k - 1) / 10  =  -k / -12

         -12k + 12  =  -10K

                     12  =  2k

                       6  =  k