Use the Naegele's Rule
The Naegele's rule is based on a regular 28 days cycle and it estimates the EDD as follows; LMP - 3Months + 7 days.
Since your patient is have 34 days cycle then you have to add the extra days 34-28= 6 days. So the formula will be
LMP - 3 Months + 13 days.
Always remember that the luteal phase is always 14 days.