Which of the following equations is a quadratic ?

1. x³ - x² - x + 5 = 0
2. x⁴ - 10
3. 7x² = 49
4. x⁴ - x³ = 9000
5. None of the above

Clearly, 7x² = 49 or 7x² - 49 = 0, which is of the form ax² + bx + c = 0, where b = 0.
Thus, 7x² - 49 = 0 is a quadratic equation.