We have: .
I've got this problem from my SAT Math level 2 prep book, which I think has the wrong answer.
The question is:
So what I did was subtract the matrix on the right by the left, and then both multiply both sides by -1.
The answer I get is
(-4 1 -11)
8 -1 -2
However in the book, the answer says b)
I have never done matrices before reading this book, and I'm trying to learn from the book. This is proving difficult as this book is quite hard to read, although I correctly answered the previous questions.
because matrix arithmetic involves a lot of arithmetic operations "all at once", it is not uncommon to make various sign errors or other simple arithmetic mistakes in the process of doing one's calculations. i should know, this happens to me all the time.
so one must be extra vigilant in checking and re-checking your work.
it seems clear that what happened, is you added 8, instead of subtracting it (in the upper-right hand corner), other than that, everything is fine. your logic was fine, there was an error in execution (a sign error).