I am taking the Low Code App Builder Mission and am stuck on the calculating values module quiz on the following question: A ticket seller requires a calculated field to find the total cost of tickets purchased, sales tax, and processing fee. Complete the calculation configuration by entering the missing operations. 

.SubTotal              .Tax              .ProcessingFee - where I need to fill in the space after sub total and after tax. 

I have read and re-read the information and done the create a calculation practice and yet I am stuck.  Can anyone point me to somewhere that I can get some information to figure out the correct answer to this?   I am new to Pega and have no development experience.   Thank you.