im trying to learn c++ on my own but i dont understand some of the things the question is asking im still a beginner

[CoreyS18]

Write an interactive program which allows the user to enter values into subscripted variable 'x' of length n determined by the user. Also the program should displays all elements with even subscripts only all horizontally and calculates and displays the sum of the element with subscript only

[keskiverto]

subscripted variable 'x' of length n determined by the user

An array, whose size is determined during runtime. The C++ Standard Library
provides std::vector. That has subscript operator and dynamic size.

See: http://www.cplusplus.com/reference/vector/vector/vector/

i dont understand some of the things

You should list the exact things.