Select all of the following that are quadratic equations.

a. x^2 - 2x = 4x + 1
b. x^3 - 6x^2 + 8 = 0
c. 5x - 3 = 0
d. 2x^2+ 12x = 0
e. 5x - 1 = 3x + 8
f. 9x^2 + 6x - 3 = 0