Answer:
The x-coordinate is \dfrac{\pi}{6}[/tex].
Step-by-step explanation:
We are given a function f(x) as:

Now on differentiating both side with respect to x we get that:

When 
this means that 
Hence, cosine function takes the negative value in second and third quadrant but we have to only find the value in the interval
.
also we know that
----(1) (which lie in the second quadrant)
so on comparing our equation with equation (1) we obtain:

Hence, the x-coordinates where
for
is
.