After arranging the terms in descending powers of a variable, select the opposite of the polynomial. 9 - 12n 2 + 4n 3 - 7n

So first you put the descending powers of a variable in order. So 4n^3 - 12n^2 -7n +9. Now that you have them in descending order you do the opposite sign of every term. So your answer will be   -4n^3 +12n^2 +7n -9. Hope this helps. 

  this would be the correct answer
-4n^3 +12n^2 +7n -9

This polygon has 4 sides
180(4-2) = 180(2) = 360 degree
80+90+2x + 3x = 360
170+5x = 360
5x = 190
x= 38
measure of angle t is 2(38)= 76
the answer is c. trust me
hope this !