If three points a (1,5) B (a, - 2) C (- 2, - 1) are collinear, then the value of real number a is Thank you

If three points a (1,5) B (a, - 2) C (- 2, - 1) are collinear, then the value of real number a is Thank you

kAB=7/(1-a)
kAC=6/3=2
kAB=kAC
7/(1-a)=2
1-a=7/2
a=-5/2