25x2−20x−21=025 x^{2}-20 x-21=025x2−20x−21=0
25x2−20x+21=025 x^{2}-20 x+21=025x2−20x+21=0
25x2+20x−21=025 x^{2}+20 x-21=025x2+20x−21=0
25x2+20x+21=025 x^{2}+20 x+21=025x2+20x+21=0
