The bottom next number is -2.28 and the equation becomes 
 , where n is the input value and O is the output value.
, where n is the input value and O is the output value.
Explanation:
The given values are,
 Input Output
 1 2
 2 0
 3 -2
 4 -4
 
 ?
 ?
Step:1
 Based on the input and output values assume the equation,
  
 .....................(1)
.....................(1)
 Where,
  n - Input values
 O - Output values
Step:2
 Check for solution,
 Take, n = 1
 O = 2 ( 2 - 1 )
 = 2 ( 1 )
  O = 2
 Take, n = 2
 O = 2 ( 2 - 2 )
 = 2 ( 0 )
  O = 0
 Take, n = 3
 O = 2 ( 2 - 3 )
 = 2 ( -1 )
  O = -2
 Take, n = 4
 O = 2 ( 2 - 4 )
 = 2 ( -2 )
 O = -4 
 Now, we substitute the value for n = 
 ,
,
 Take, n = 

 O = 2 ( 2 - 
 )
 )
 = 2 ( 2 - 3.14 ) ( ∵ 
 = 3.14 )
 = 3.14 )
 = 2 ( -1.14 )
  O = -2.28
Result:
 The bottom next number is -2.28 and the equation becomes 
 , where n is the input value and O is the output value.
, where n is the input value and O is the output value.