function search(m, x)
   for i,v1 in ipairs(m) do
      for j,v2 in ipairs(v1) do
         if v2 == x then return i,j end
      end
   end
end

m = {
   {1,2,3,4,5},
   {6,7,8,9,10},
   {11,12,13,14,15},
   {16,17,18,19,20},
   {21,22,23,24,25},
}

local i,j = search(m,17)
print(string.format("m[%d][%d] = %d",i,j,m[i][j]))

m[4][2] = 17