I have a 1099R Roth and trying to figure the box 2a?

"I just need to know how to calculate the taxable portion."

Which means there are more details that have not been revealed here to the internet, but that you will find out and apply to what you read, as I described. For instance, your taxpayer's age? Is he/she still employed there? Disabled? Retired? Dead? Did you at the least scan the links provided, such as reading the section:

"What types of distributions cannot be qualified distributions and must be included in gross income?

You cannot treat the following types of distributions from a designated Roth account as qualified distributions (or eligible rollover distributions) and must include any earnings paid out in gross income:" in the first link I offered. And:

"A qualified distribution of designated Roth contributions is excludable from gross income. A qualified distribution is one that is made at least five years after the year of the participant's first designated Roth contribution (counting the first year as part of the five) and is made:

• On or after attainment of age 59½,
• On account of the participant's disability, or
• On or after the participant's death."

Which is part of the second link I offered.

You won't need to "do the math." You need to know eligibility, exception, basis, etc for your taxpayer. Here is the article for how to enter the 1099-R:

https://accountants.intuit.com/support/en-us/help-article/form-1099-r/entering-form-1099-r-proseries/L0otcKvVi_US_en_US?uid=lubt1sro