Question

x=2a-1..................

Answer 1

$a\in\{1,2,3,4,5,6\} \\ \\ 2a\in\{2,4,6,8,10,12\} \\ \\ 2a-1\in\{1,3,5,7,9,11\} \\ \\ x\in\{1,3,5,7,9,11\}$

Answer 2

x =2a -1
a ∈ { 1 ;2 ;3 ; 4 ; 5 ;6 } *2
2a ∈ {2 ;4 ;6 ;8 ;10 ;12 }
2a-1 ∈{1 ;3 ;5 ;7 ; 9 ;11 }
x ∈ {1 ;3 ;5 ; 7 ;9 ;11 }