Answer
$a(x-x_1)(x-x_2)=0$, where $a$ is any real number
Work Step by Step
Let's note that $\{x_1,x_2\}$ is the solution set of a quadratic equation.
Because $x_1$ and $x_2$ are solutions of the equation, it means the equation has the factors $x-x_1$ and $x-x_2$. Therefore the general form of such an equation is:
$$a(x-x_1)(x-x_2)=0, \text{ where }a\text{ any real number}.$$
Example
Take $\{2,7\}$ as the set of solutions of a quadratic equation. So $x-2$ and $x-7$ are factors of the equation:
$$a(x-2)(x-7)=0$$
Since $a$ is any real number, take for example $a=1$. The equation is:
$$(x-2)(x-7)=0$$
$$x^2-9x+14=0.$$
