for finding the perimeter of abc (triangle), we MUST need the two others coordinates of b and c
let b(1, 2) and c (2, -4)
we need to calculate the distances ab, ac, and bc
vector (ab)=(1-(-2), 2-9)=(3, -7), so its length is ab=sqrt(3²+ (-7)²)=7.61
realising the same method, we find bc=6.08, ac=13.60
so the perimeter of abc is P= ab+bc+ca=13.60+6.08+7.61=27.29