How did the 9/11 attack affect the USA ?

In a recent survey, 93% of Americans ages 30 and above said they can remember exactly where they were or what they were doing the moment they learned of the terrorist attacks on September 11, 2001.  During the past six decades, only the Kennedy assassination had such a pervasive and enduring impact. It is hardly surprising, then, that in 2016, more than three-quarters of American adults named 9/11 as a top historical event of their lives, nearly twice as many as for the second most-cited event.

Explanation:

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