Use the chain rule to get the derivative (the slope), then use the given point to write the equation.
y = sin(sinx)
dy/dx = cos(sinx) cos(x)
x = 3π
sin(3π) = 0
cos(sin(3π)) = cos(0) = 1
cos(3π) = -1
m = dy/dx|x=3π = -1
y = -x + b
0 = -3π + b
b = 3π
y = -x + 3π