Question

Решите плиз даю 50 балов!!!

Answer 1

$\displaystyle\bf\\3x(7x-2)-2x(9x+7)=21x^{2} -6x-18x^{2} -14x=\\\\\\=\underbrace{(21x^{2} -18x^{2} )}_{3x^{2} }-\underbrace{(6x+14x)}_{20x}=3x^{2} -20x\\\\\\x=-4\\\\\\3x^{2} -20x=3\cdot(-4)^{2} -20\cdot(-4)=3\cdot 16+80=\\\\\\=48+80=128\\\\\\Otvet \ : \ 128$