You can obtain the answer . . . . . . . . adding the date of birth to this figure.

In this context, the appropriate preposition to use is "by".

The sentence "You can obtain the answer by adding the date of birth to this figure." suggests that the answer can be obtained through the action of adding the date of birth to the given figure.

The preposition "by" is commonly used to indicate the method or means by which an action is performed. In this case, the action of obtaining the answer is performed by adding the date of birth to the given figure.