Since the "b" row is missing "c" and "d", and since "bc" cannot equal "c", it follows that "bc" = "d", and therefore "bd" must equal "c". Multiplying on the right by "f" this gives us "b" = "cf", which we can further manipulate into "cb" = "f" by multiplying by "c" on the left. By similar logic we can deduce that "c" = "fb" and that "dc" = "b". Filling these in, we have (with the latest additions in green):