-9.39% is the change from 309 to 280
Calculation steps:
[( 280 - 309 ) ÷ 309] x 100 = -9.39%
Formula
Percentage change = [(280 - 309) / 309] x 100
Subtract the new number by the original number
280 - 309 = -29
-29 ÷ 309 = -0.09385113268608414
-0.09385113268608414 x 100 = -9.39%