[tex]\displaystyle\bf\\x^{2} +(a-2)x+28-4a=0\\\\Teorema \ Vieta \ :\\\\x_{1} + x_{2} =-(a-2)=2-a\\\\x_{1} \cdot x_{2} =28-4a\\\\\\2-a=28-4a+1\\\\-a+4a=29-2\\\\3a=27\\\\a=27:3\\\\\boxed{a=9}[/tex]