Use of uninitialized value $Conclusion[2] in concatenation (.) or string at Unit3.pm line 203, <STDIN> line 12.
c => -b =>  => 

or 

Use of uninitialized value $OtherConclusion[1] in concatenation (.) or string at Unit3.pm line 210, <STDIN> line 12.
Use of uninitialized value $Conclusion[2] in concatenation (.) or string at Unit3.pm line 214, <STDIN> line 12.
f =>  => -b

In other words, c =>  or f => -b