\[На\ отрезке\ \lbrack - 2;\ 0\rbrack:\]
\[y = x^{3} - 3x + a;\]
\[y^{'}(x) = 3x^{2} - 3 \geq 0;\]
\[x^{2} - 1 \geq 0\]
\[(x + 1)(x - 1) \geq 0\]
\[x \leq - 1;\text{\ \ \ x} \geq 1.\]
\[Точка\ максимума:\]
\[y( - 1) = - 1 + 3 + a = 5;\]
\[a = 5 - 3 + 1 = 3.\]
\[Ответ:\ \ a = 3.\]