\[y = - 3x^{2} - 6x + 5;\ \ x \in \lbrack - 2;1\rbrack\]
\[y( - 2) = - 3 \cdot 4 + 12 + 5 = 5;\]
\[y( - 1) = - 3 \cdot 1 + 6 + 5 = 8;\]
\[y(1) = - 3 - 6 + 5 = - 4.\]
\[D(y) = \lbrack - 4;8\rbrack.\]