Why would someone spell as pie?. You can't really think it's spelled that way?.

It's not a pastry, it's a letter of the Greek alphabet. PI

There's different methods to find the solution to #1. Calculus or non-calculus.

The height of the empty region in the tank is given by

Solving for theta, we find

The volume of a circular segment is given by

The length of the tank is 6, so the volume of this portion of the tank is

Subtract this from the volume of the tank which is

and get

Then use the volume of a cylinder formula and solve for h:

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Calc method:

Now, the same method as before and we get the same answer.