Its very easy. Just concatenate two strings.
In the test cases you wrote "your_fcn_name(x)" and not "your_fcn_name(x1,x2)" ;-)
Fixed it.
Duplicate of 2081
This user (ALBERT ALEXANDER STONIER) appears to have gamed the system, with fraudulent "likes" of this unremarkable solution submitted by 'sock-puppet' accounts. —DIV
please improve test suite