Thus you have,

And I assume you mean,

So find the derivative und make it zero.

Thus,

Multiply through by the radical,

Divide by 200,

Thus,

But,

.

Thus,

Is the only solution.

Now take any point to the left of 9 (and more than zero) and see what the sign is. Positive increasing and negative decreasing. Same for any point to the right of 9 (and less than 12)