sin -1

I think you mean inverse sin. Sometimes called arcsin
NOTE: inverse sin is totally different from (sin x) ^-1 which means 1/sinx

sin 30 ^o = 1/2 = 0.5
therefore
arcsin 0.5 = 30 ^o
or it might be written as
sin ^-10.5 = 30degrees

on a calc you press
asin 0.5=
or
inv sin 0.5 =

You just have to be very careful that the calculator is displaying degrees rather than radians. This is a common error.

I just fixed a couple of little typos in my answer.
It might make more sense now.

Hi you should see sin of your number then by putting negative mark you can solve your problem like this :
Sin-90=sin 90=1
Sin -90=-(1)=-1

If you just wanted
sin(a negative angle) Mujib is right.

sin(-theta) = -sin(theta)

I don't think that this is what you were asking.