`D = [-1; +infty)`
`sqrt{3x + 7} - sqrt{x + 1} = 2`
`-> (sqrt{3x + 7})^2 = (2 + sqrt{x + 1})^2`
`-> 3x + 7 = 4 + 4sqrt{x + 1} + x + 1`
`-> 2x + 2 = 4sqrt{x + 1}`
`-> x + 1 - 4sqrt{x + 1} = 0`
`-> sqrt{x + 1)(sqrt{x + 1} - 4) = 0`
`->` \(\left[ \begin{array}{l}x + 1 = 0\\x + 1 = 16\end{array} \right.\)
`->` \(\left[ \begin{array}{l}x = -1\\x = 15\end{array} \right.\)
`-> S = {-1; 15}`
