Problem with a root. - path

hi i have a quick question what is the syntax if u want to use parametrs to define a path inside a path? i mean:
_root[“mc1”+n][“mcIn1”+a]._x = …
or
_root[“mc1”+n[“mcIn1”+a]]._x = …
or
_root[“mc1”+n].[“mcIn1”+a]._x = …
or
_root[“mc1”+n.“mcIn1”+a]._x = …
how do u write the path?!

does anyone know…
it’s very important!!!
any replie would be very appreciated!!!
thanks ahead…