Why do you think the mormons were confident that they could prosper in a new environment when they migrated west?

Because there was not a large Native American presence, but there was the potential for agriculture, and for supporting a large population."

Explanation:

After the murder of Mormons founder and prophet, Joseph Smith,

in Illinois, Brigham Young took over the leadership of Mormon and lead them to Utah due to continues mob attack. Though, the settlement is salt lake but Mormon sees an opportunity in becoming the majority since there was limited numbers of native American and also they were able to practice agriculture.

Since there was not a large Native American presence, and for supporting a large population there was the potential for agriculture.

Explanation:

After the murder of Joseph smith, the founder and prophet of Mormons Brigham Young, in Illinois, became a leader of Mormon and lead them to Utah due to continues mob attack.

Although, the settlement is salt lake but also, they were able to practice agriculture and they (Mormon) saw an opportunity in becoming the majority since there was limited numbers of native American.