"Box With No Top" Optimization

"Box With No Top" Optimization

A box with no top is to be constructed from a piece of cardboard of
dimensions A by B by cutting out squares of length h from the corners and
folding up the sides as in the figure below. Suppose that the box height
is h = 1 in. and that it is constructed using 186 inches^2 of cardboard
(i.e., AB = 186). Which values A and B maximize the volume?
Here is the associated image:



My Attempt:
We have, $$SA=AB+1(A+B)=AB+2A+2B=186.$$
Solving for $A$,
$$A=(186-2B)/(B+2).$$
Now, use this to calculate the volume,
$$\begin{align} V &= ABH \\ &= AB \cdot 1 \\ &= AB \\ &= \left(
\frac{186-2B}{b+2} \right) B. \\ \end{align}$$
Differentiating with respect to $B$,
$$\frac{dV}{dB} = \dfrac{-2(B^2+4B-186)}{(b+2)^2}.$$
Setting $\frac{dV}{dB}$ equal to zero and solving for $B$ yields,
$$B= -2-\sqrt{190} \ \text{ or } \ B = \sqrt{190}-2.$$


However, neither of these solutions are correct. What am I doing wrong?