An inverted conical water tank with a height of 12ft and a radius of 6ft is drained through a hole at the vertex at a rate of 2ft^3 / s .

1. An inverted conical water tank with a height of 12ft and a radius of 6ft is drained through a hole at the vertex at a rate of 2ft^3 / s . What is the rate of change of the water depth when the depth is 3ft ? (Hint : use similiar triangles)