Glad you figured it out on your own, because the phraseology "I get only part marks" indicates that this problem is for a grade. It is forum policy not knowingly to help with such problems.
This is the question i am trying to solve...
http://img151.imageshack.us/img151/7596/mathprob.jpg
U is a 3x3 matrix.
However, when i put my answer in I get only part marks... 78% right.
My attempt:
I found that to get B, I had to use elementary row operations. Mine were:
-2r1 + r2 --> r2
and r1 + r3 --> r3
after that I used an identity to find U, and i applied the same row operations.
My matrix/U looked like this after:
1 0 0
-2 1 0
1 0 1
What am i doing wrong???
UPDATE:
I just figured it out...
I just had to invert U to get my answer