Select all of the following that are quadratic equations.

5 x 2 + 15 x = 0
6 x - 1 = 4 x + 7
x 2 - 4 x = 4 x + 7
2 x - 1 = 0
3 x 2 + 5 x - 7 = 0
x 3 - 2 x 2 + 1 = 0