Since h = 1/2 r then you'll have r as:
r = 2h
note that the formula in finding the volume of a cone is defined by:
where r in that case is 2h. then you'll have it as:
since you are to find the rate at which the height is increasing then you will have to differentiate the derived equivalent formula.
substituting the given you'll have:
Given: dV/dt = 1cu.m/min
h = 6m