Question 808932
Let
(1) sin(x) = y
Then
(2) x = arcsin(y)
Put y of (1) into (2) and see that
(3) x = arcsin(sin(x))
In your problem
(4) x = -3pi/5
So the exact value of 
(5) arcsin(sin(-3pi/5)) = -3pi/5
When the exact answer is asked for, use the symbol, pi in this case, for the irrational number not a decimal.