The simplest solution is to simply check for equality:
idx = x==Matrix_A;
which will return a logical array with true in every location where the value x occurs in that matrix.
If you want to convert that info to subscript indices, then you can use find on that logical array:
[R,C] = find(idx)
In many cases using a logical index is simpler and more efficient, but it really depends entirely on what you are going to do with these indices, which so far you have not explained.
