Answer :
o = 2i - 1
o = output
2 i = twice the input
then -1 = one less
o = output
2 i = twice the input
then -1 = one less
If the output equals o, and the input equals i, the equation would be o = 2i - 1, because twice the size of the input is 2i, one less than is -1, and o is the output.
