How do you know if a function is quadratic??

F(x) = y = ax^2 + bx + c is the equation, where a, b, and c are numbers and a is not equal to zero. If it was zero, then the equation would be linear and not quadratic. For example, if you have f(x) = x +9 + 4x^2, you would rewrite the function with the largest exponent first, so you would have f(x) = 4x^2 + x + 9.