% given matrix with three rows:
M = [ ...
1 2 3 4 5 6; ...
3 1 2 2 3 1; ...
2 3 3 1 2 3];
% value to look for in the second row of M:
val = 2;
% find the indices where val is in the second row of M:
idx = find(M(2,:) == val);
% output is val followed by those indices, all together in a column vector:
output = [val; idx(:)]
