Water is dripping out from a conical funnel of semi-vertical angle at the uniform rate of in the…

Water is dripping out from a conical funnel of semi-vertical angle $\frac{\pi}{4}$ at the uniform rate of $2\text{ cm}^2/\text{sec}$ in the surface, through a tiny hole at the vertex of the bottom. When the slant height of the water level is $4\text{ cm}$, find the rate of decrease of the slant height of the water.