Given the points $\mathrm { A } ( 1 , - 2,3 ) , \mathrm { B } ( 0,2 , - 1 )$ and $\mathrm { C } ( 2,1,0 )$. Find:\
a) (1.25 points) Verify that they form a triangle $T$ and find an equation of the plane containing them.\
b) ( 0.75 points) Calculate the intersection of the line passing through points A and B with the plane $z = 1$.\
c) ( 0.5 points) Determine the perimeter of triangle T.