Sorry for asking such a basic question.
In the following quadratic equation
$$x^2+2(a-3)x-3a-7=0$$
by my calculations,
$$D=\left(\frac{b}{2}\right)^2-ac=(a-3)^2-1(-3a-7)=a^2-6a+9+3a+7=a^2-3a+16$$
But in a PDF article that describes a solution to a problem concerning this equation the discriminant is calculated as
$$a^2-3a+2$$
Is there any error in my calculation?
Notice, compare the given equation: $x^2+2(a-3)x-3a-7=0$ with $Ax^2+Bx+C=0$, we get $$A=1, \ B=2(a-3), \ C=-3a-7$$
Hence,
discriminant is calculated as follows $$D=B^2-4AC=\left(2(a-3)\right)^2-4(1)(-(3a+7))$$ $$=4a^2-24a+36+12a+28$$ $$=4a^2-12a+64$$