21t² - (7t - 3) × ( 3t + 1 ) = -2
21t² - ( 21t² + 7t - 9t - 3 ) = -2
21t² - 21t² - 7t + 9t + 3 = -2
21t² - 21t² - 7t + 9t = -2 - 3
2t = -5
t = (-5) ÷ 2
t = -2,5.
ответ : t = -2,5.
Объяснение:
