I beilive I have heard that a decagon equals 1440 degree's what you can do lets see you have 9 angles degree mesaurments on here add them then subtract the sum from 1440 after that your diffrence will be your answer.....

like this

\(156 + 178 + 124 +132 +138+142+116+178 +159 = sum\)

\( 1440 - sum = difference \)

\(Difference = answer.\)