What did President James Polk do in hopes of gaining additional territory, including California and New Mexico?

1. ask Congress for a formal declaration of war on Mexico
2. appoint Stephen Austin to serve as ambassador to Mexico
3. call for a treaty with Mexico
4. agree to accept the Nueces River as the southern border of Texas

3 - call for a treaty with Mexico

Explanation:

President James Polk called for a treaty with the Mexican government which was called as Treaty of Guadalupe. This treaty ended the Mexican-American war that took place during 1846-48.

Gaining the territories of California and New Mexico from Mexico was one of the major goals of President James.

Mexico signed a treaty with America in which it recognized Rio Grande southern border of America. In the same treaty, Mexico ended up with giving California, New Mexico, Arizona, Colorado, Utah, Nevada and Wyoming to America in exchange of $15 million.