\[\sqrt{x} = a - 2\]
\[x = (a - 2)²\]
\[a = 2 \Longrightarrow x = 0;\]
\[a < 2 \Longrightarrow нет\ решения.\]
\[a > 2 \Longrightarrow x = (a - 2)^{2}.\]