MATLAB: How to find the index of a known number? (reverse indexing?)

indexmatrixnumberrandomvector

I have a matrix M.

I have a number XXXX.

I want to find the index of this number within my matrix M.

How can I do this?

idx = find(M==xxxx)

matrix(20)

idx = 1:numel(A);
mask = A < 5;
idx = idx(mask);