We cashed out an inherited Roth IRA from father. box 2a on 1099-R is blank and box 2b is checked. box 4 is 0.00 and box 7 is Q. how do i figure out taxable amount?

None of it is taxable, if your father open the Roth more than 5 years ago. Enter the 1099-R and the interview will guide you. 

Reference: https://www.rothira.com/roth-ira-beneficiary-rules

Code Q indicates that the payer knows that the account was held for 5 years or more.  The distribution is not taxable.  The distribution is not reportable on any Form 8606.  Entering the Form 1099-R into TurboTax will result in the gross amount appearing on Form 1040 line 15a or Form 1040A line 11a but it will be absent from line 15b or 11b, making it nontaxable.