I have some questions that I have the answers for but no idea how to reach those answers.

1)

x^3 - 6x^2 +5x + 6 < 0 (use sign graph)

the < is supposed to be "less than or equal to".

the answer I'm given is: (-inf, 2-sqrt7] U [2, 2+sqrt7]

I've never learned what that way of writing the answer means though.

2) find f-1(x) (it's supposed to be find the inverse)

f(x) = -(x-2)^2, x < 2

the < is supposed to be "less than or equal to".

the answer I'm given is: f-1(x) = 2-sqrt(-x)

3) find f-1(x) (it's supposed to be find the inverse)

f(x) = (2x+1) / (x+3), x cannot equal 3.

the answer I'm given is: f-1(x) = (1-3x) / (x-2)

-inf means negative infinity, sqrt means square root of

Please help me.