[tex]\displaystyle\bf\\x^{2} +15x+36=0\\\\Teorema \ Vieta:\\\\\boxed{\boxed{x_{1} +x_{2} =-15}}\\\\\boxed{\boxed{x_{1} \cdot x_{2} =36}}\\\\\\x_{1}=-3\\\\x_{2} =-12\\\\\\x_{1} -x_{2} =-3-(-12)=-3+12=9\\\\\boxed{\boxed{x_{1} -x_{2} =9}}[/tex]