In the Use Numbers, Currency, and Percentages in Formulas unit, I cannot for the life of me get the challenge to work. My formula is right (I've tried multiple versions which all should work, including one with rounding and using the bracketed exponent) my formula field is set to 0 decimal places, I've deactivated required fields and old validation rules, and my spelling is correct. I have absolutely no idea where to go from here. Does anyone have any ideas?

Field Label: Cylinder Volume
Object Name: Opportunity
Field Name: Cylinder_Volume  
API Name: Cylinder_Volume__c
Data Type: Formula  
Decimal Places: 0  

3.14159 * radius__c * radius__c * height__c

Your company sells cylindrical hyperbaric chambers. You need a formula field that calculates the volume of a cylinder for you, rounded to the nearest whole number, given its radius and height. Use the existing mathematical formula for the volume of a cylinder, V = πr2h, where r is the radius of the cylinder, h is the height, and π is the constant Pi. Note: Although this formula field might best be created on a custom object, for simplicity, we’ll create this formula on the Opportunity object.
>Create 2 custom fields of type Number on the Opportunity object: ‘radius’ with a resulting API name of ‘radius__c’ and ‘height’ with a resulting API name of ‘height__c’.
>The formula should be named ‘Cylinder Volume’, with the resulting API name ‘Cylinder_Volume__c‘ and should be created on the Opportunity object.
>The formula should reference the custom fields ‘radius__c‘ and ‘height__c‘.
>The formula should use 3.14159 as an approximation of Pi.

Heres the challenge that im stuck on. i have created the 2 custom fields. What im stuck on is where do we create the formula. Do we put it in the custom fields or is there another place u insert it?

thanks if anyone can help