A quadratic equation is an equation of the form ax 2 + bx + c = 0, where a, b and c are constants. For example 3x 2 + 2x - 9 = 0 is a quadratic equation with a = 3, b = 2 and c = -9. The constants b and c can have any value including 0. The constant a can have any value except 0. This is to ensure that the equation has an x 2 term.