P = (1,1,1), Q = (1,0,1), R = (0,2,2)
u = Q - P = (0,-1,0)
v = R - P = (-1,1,1)
Then,
r = r0 + su + tv = r0 + s(0,-1,0) + t(-1,1,1)
which is not
r = (1,1,1)+s(1,0,1)+t(0,2,2)?????
_____
Anyway, I'm not doing the computation. If you did similar to my explanation to b), then it's right...