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honorstrigstudent
Your mistake was that you wrote 7x-5 and 9x-16 as 7x+5 and 9x+16. You have to add the expressions the way it is. It helps to put the parenthesis around it. So when you add (7x-5)+(9x-16)+(3x+11) you get 19x -10 =180. So 19x=190, so x=10.
You have the right idea: add up the three angles and set the sum equal to 180. However you misplaced a few signs. You wrote 7x+5 when it should be 7x-5. Also you wrote 9x+16 when it's actually 9x-16. 

Think of the three angles as A, B, C
A = 7x-5
B = 9x-16
C = 3x+11
Adding up A,B,C will lead us to 180 degrees
A+B+C = 180
7x-5+B+C = 180 ... replace A with 7x-5
7x-5+9x-16+C = 180 ... replace B with 9x-16
7x-5+9x-16+3x+11 = 180 ... replace C with 3x+11

Now we solve for x
7x-5+9x-16+3x+11 = 180
 19x-10 = 180
 19x-10+10 = 180+10 ... add 10 to both sides
 19x = 190
 19x/19 = 190/19 ... divide both sides by 19
 x = 10
 
 Answer: 10


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