Question

Please help me 18-20 please Thanks

Answer 1

18. American expansionists cried "Fifty-four forty or fight" in reference to the line to which they wanted their northern territory to extend.
This entire event is referred to in history as the Oregon boundary dispute/Oregon question, which relates to the division of North America. Expansionists obviously wanted America to thrive and expand to other countries which didn't originally belong to it, which led to many wars with Native Americans, as well as the Spanish who had previously acquired some countries in South America in particular, and in this case, with Canada.

19. The correct answer is water.
Given that the West was a particularly arid area due to the dry climate, the issue of water, or rather the lack of it, has always been an important one. There were many attempts to implement laws about water because this area has always been lacking in it. The issue of equality wasn't really that important because nobody could think about that if they didn't have water which is a basic need for every living being if they want to keep on living.

20. The correct answer is Mariano Guadalupe Vallejo.
Vallejo was an important Californio (a Spanish descendant living in California) who wanted America to take over in California, instead of Spain and Spanish rule. Even though he was a citizen of Mexico initially, he still believed that California was better of having America as its ruler than Spain. Thanks to his achievements during this part of American history (19th century), a city in California (Vallejo) was named after him in his honor. 

Answer 2

Fifty-four forty or fight! is the Answer, you should study more about history