Why were many federal troops placed in western texas after the civil war?

In 1866, after the Civil War, Federal troops were placed in Western Texas in order to restore order, ensure government loyalty and enforce the freedom slaves had just acquired as a result of the war. The U.S military entered the state because commanders believed this was the best way to protect the rights of the blacks and to support president Abraham Lincoln's decision.  

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simplify brackets

8 - 2a - 7 = a + 40

simplify 8 - 2a - 7 to -2a + 1

-2a + 1 = a + 40

add 2a to both sides

1 = a + 40 + 2a

simplify a + 40 + 2a to 3a + 40

1 = 3a + 40

subtract 40 from both sides

1 - 40 = 3a

simplify 1 - 40 o -39

-39 = 3a

divide both sides by 3

-39/3 = a

simplify 39/3 to 13

-13 = a

switch sides

a = -13