Ok, so I haven't checked your calculations for a), but your method is right
Right, part b).
You've got the equation for voltage, you worked it out to be v = 0.0412t^2 + 0.0002t^3. You want to find the minimum of this. So if you were to plot a graph for voltage, the minimum would be the at the lowest point, where the gradient is zero. So you've got an equation for the gradient, you worked out
dv/dt = 0.0824t + 0.0006t^2. You need to let this equal zero and solve for t. If you get more than one value out for t, work out which one is the minimum (the other will be a maximum).
Hope this helps.