(а-3)(2а+7) = 2а²+7а-6а-21 = 2а²+а-21
а(2а+1)-2(а-1)+а(а+1) = 2а²+а-2а+2+а²+а = 3а²+2
2а²+а-21 + t = 3а²+2
t = 3a²+2-2a²-a+21
t = a²-a+23.
(а-3)(2а+7) = 2а²+7а-6а-21 = 2а²+а-21
а(2а+1)-2(а-1)+а(а+1) = 2а²+а-2а+2+а²+а = 3а²+2
2а²+а-21 + t = 3а²+2
t = 3a²+2-2a²-a+21
t = a²-a+23.