For the record, I stand behind my close vote on this question. This is by no means a "clear abuse of authority", as illustrated by the fact that four others voted to close, nine people downvoted it, and three others voted to delete the question. The community clearly found something wrong here.
Several things smell about this "question". First, the title:
Write a program to multiple a matrix of size (n*m) in java?
while containing a question mark, has the tone of asking us to do the user's work for them. People respond very poorly to being commanded to do someone's job / homework for free.
The body of the question:
I just want to know how to multiply two matrix(n*m).
is just a single line, with no question in it. It's also asking for a common task whose implementation can be found elsewhere. As was pointed out in this Meta question the "not a real question" close reason is being applied to trivial / general reference questions, but a new close reason may be created for these based on some of the confusion around the "not a real question" wording.
The user also doesn't provide any context for what they're trying to do, the constraints on their problem, etc., so it could be considered to be too broad to answer exactly.
The overwhelmingly negative response this question received most likely comes from the fact that we see piles of these sort of lazy, "do my work for me" questions come in here every day. These are the kinds of questions that cause experts to get fed up and leave, and Stack Overflow is structured to prevent the kinds of collapses due to this that have occurred in almost every other programming-related mailing list or forum I've participated in. We're doing what we can to keep the signal to noise ratio high here as the site scales to an incredible amount of question and answer traffic. That sometimes means we prune questions like this to maintain the overall site quality.
If I may comment on the negative response to your answer, two things bothered people there. First, spoon feeding answers to help vampires like this is starting to get frowned upon here, because that just encourages others to come here and have others do their work for them.
The second, and most important, criticism people had was that you just copied and pasted someone else's code without checking it yourself or providing any summary of what it does. As others commented, it turned out to be broken, which is harmful to have in an answer. Far too many people copy and paste code from Stack Overflow into their projects, so having buggy code here can be a real problem.
So no, this wasn't an "abuse of authority", this was an unsalvageable question and it deserved to be closed and deleted.