Please answer fully and clearly. Explain all conclusions

(3) You have a 4x4 square grid with -1's in the top row, -1's in the leftmost column, and +1's everywhere else. You can change the sign of all the entries of any row or any column on each step. Show that you can never make this matrix be entirely positive (i.e., make all its entries +1. -1 -1 -1 -1 -1 +1 +1 +1 -1 +1 +1 +1 +1 +1 +1+ 11 -1 +1