Select all of the following that are quadratic equations. 3x 2 + 5x - 7 = 0 x 3 - 2x 2 + 1 = 0 2x - 1 = 0 5x 2+ 15x = 0 6x - 1 = 4x + 7 x 2 - 4x = 4x + 7