Answer:
the answers are:
1-upsetting balance between Free and Slave states.
2-a treaty of annexation
3-In December of 1845, Texas became the 28th state in the U.S.
Step-by-step explanation:
1-The main opposition was found in the US. The issue of slavery was central to their concerns. Abolitionists in the U.S. worried that adding another slave-holding state would upset the political balance in Congress and in the country.
2- In 1836 Texas declared its independence from Mexico but, It was not until eight years after Texas gained independence from Mexico that President John Taylor succeeded at annexing Texas through a Treaty of Annexation.
3-In 1845, the Republic of Texas was annexed to the United States of America, becoming the 28th U.S. state.