y' = dy/dx = d[e^cos(3x)]/dx = [e^cos(3x)] * d[cos(3x)]/dx = [e^cos(3x)] * [-sin(3x)] * d(3x)/dx
y' = -3 * sin(3x) * e^cos(3x)
y' = dy/dx = d[e^cos(3x)]/dx = [e^cos(3x)] * d[cos(3x)]/dx = [e^cos(3x)] * [-sin(3x)] * d(3x)/dx
y' = -3 * sin(3x) * e^cos(3x)