The search results do not provide sufficient information about a U.S. nonprofit organization named PTA Delaware Congress with the EIN 521335766 and the website mutualpta.com. Therefore, it is not possible to write a Wikipedia-style article based on the available information. If you are referring to the Delaware State PTA, here is a general description:

Delaware State PTA

The Delaware State PTA, also known as the Delaware Congress of Parents and Teachers, is a registered 501(c)(3) non-profit organization. It is the largest and oldest child advocacy organization in Delaware, comprising over 7,000 members. The organization serves as a powerful voice for children, a relevant resource for parents, and a strong advocate for public education. Membership is open to anyone concerned with the education, health, and welfare of children and youth, aligning with the mission of the National PTA.

However, the specifics for PTA Delaware Congress with the EIN 521335766 are not available in the search results.