\[\boxed{\text{86\ (86).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
Пояснение.
Решение.
\[\textbf{а)}\ да:\]
\[(2a) \cdot (7b) = 2 \cdot 7 \cdot a \cdot b = 14ab.\]
\[\textbf{б)}\ да:\]
\[- 2a + 2a = 2a - 2a = 0.\]
\[\textbf{в)}\ нет:\]
\[y - x = - (x - y)\]
\[y - x \neq x - y.\]
\[\textbf{г)}\ да:\]
\[(x - y)^{2} = x^{2} - 2xy + y^{2}\]
\[(y - x)^{2} = y^{2} - 2xy + x^{2} =\]
\[= x^{2} - 2xy + y^{2}\]
\[(x - y)^{2} = (y - x)^{2}.\]