[tex]\displaystyle\bf\\x^{2} -4x+p=0\\\\2x_{1} +x_{2} =1\\\\Teorema \ Vieta \ :\\\\x_{1} +x_{2} =4\\\\x_{1} \cdot x_{2} =p\\\\\\x_{2} =4-x_{1} \\\\2x_{1} +4-x_{1} =1\\\\x_{1} +4=1\\\\\boxed{x_{1} =-3}\\\\x_{2} =4-(-3)=4+3=7\\\\\boxed{x_{2} =7}\\\\p=x_{1} \cdot x_{2} =-3\cdot 7=-21\\\\\boxed{p=-21}[/tex]