The two equal sides of an isosceles triangle with fixed base $b$ are decreasing at the rate of 3 cm per second. How fast is the area decreasing when the two equal sides are equal to the base ?

Question

$\begin{array}{1 1} \sqrt {3b}cm^2/sec \\ \sqrt {-3b}cm^2/sec \\ \sqrt {3a}cm^2/sec \\ \sqrt {5b}cm^2/sec \end{array} $