If the qualified expenses are $6,000 and you take a $15,000 distribution for those expenses, then you will have $9,000 in non-qualified distributions, and the earnings portion of that $9,000 will be subject to income tax plus an additional 10% penalty. For example: let's say that at the time you take the distribution, the ratio of contributions to earnings in the account is 2:1. When you take the $15,000 distribution, $4,000 in contributions and $2,000 in earnings will be attributed to the qualified expenses, and $6,000 in contributions and $3,000 in earnings will be attributed to the non-qualified part of the distribution. You will owe tax and a 10% penalty on the $3,000 in earnings that was part of the non-qualified distribution.
Why do you want to withdraw $15,000 when the qualified expenses are only $6,000?