(2a-7)(3-a)=6а-2а²-21+7а=13а-2а²-21=-2а²+13а-21
-2а²+13а-21=0
D= b 2 - 4ac = 1, D > 0 => 2 вещественных решения, √D = 1
x 1 = -b + √D/2а = -13 + 1/2 × (-2) = 3 ,
x 2 = -b - √D/2а = -13 - 1/2 × (-2) = 3,5
