If you are going to graph x = sin(y), it will not give you a function because a single vertical line test will intersect multiple points on the graph. However, y= sin-1(x) is. So let's get the derivative of this:
dy/dx = 1/√(1-x2).
Since the original graph has multiple points at x=0, then we have to consider the derivative to be:
dy/dx = 1/±√(1-x2)
at (0,3π), you will see that the slope of the tangent line is going down. Therefore:
dy/dx = 1/±√(1-02)
dy/dx = ±1 (the slope is going down so we eliminate the positive)
dy/dx =-1
Therefore the slope of the tangent line is -1.