Here, 

Since, the power of the lens is negative i.e. -6D the type of lens is a concave lens. Its focal length is negative being a diverging lens so the power of lens is negative as well.

To calculate focal length:

For -6D:

\(\rm P \) = $\frac{1}{f}$

\(\rm or,\) \(\rm -6 \) = $\frac{1}{f}$

\(\rm or,\) \(\rm f \) = $\frac{1}{-6}$

\(\rm or,\) \(\rm P \) = \(\rm -0.1667\)

Hence, for lens with power -6D focal length is -0.1667 m or -16.67 cm.