27 = 20.5sin((2pi/15)x - (pi/2))

I did this:

27/20.5 = sin((2pi/15)x - (pi/2))

arcsin(27/20.5) = (2pi/15)x - (pi/2)

arcsin(27/20.5) + (pi/2) = (2pi/15)x

((arcsin(27/20.5) + (pi/2))/(2pi/15) = x

But when I entered ((arcsin(27/20.5) + (pi/2))/(2pi/15) into a calculator, it said "domain error".