\[y = \sqrt{7x - x^{2}}\]
\[7x - x^{2} \geq 0\]
\[7x - x^{2} = 0\]
\[x(7 - x) = 0\]
\[x = 0,\ \ x = 7.\]
\[D(y) = \lbrack 0;7\rbrack.\]