\[(a - 7)x = a² - 14a + 49\]
\[x = \frac{(a - 7)^{2}}{a - 7} = a - 7\]
\[1.\ a = 7 \Longrightarrow x - любое.\]
\[2.\ a \neq 7 \Longrightarrow x = a - 7.\]