急,高一函数奇偶性题!

当 x < 0 时 ,
-x > 0
f(-x) = -x(-x + 1) = x (x - 1)
因为f(x)是奇函数
所以f(x) = -f(-x) = -x(x - 1)
当 a ≥ 0 时
f(a) = a(a + 1) = -2
a 无解
当 a < 0 时
f(a) = -a(a - 1) = -2
a = -1 (a = 2 舍去)
综上： a = - 1