Welcome to Matlab and to this forum.
This seem to be a homework question. Therefore I will not post a solution, but hints.
At first I suggest to read the Getting Started chapters of the documentation. This is required if you want to work successfully with such a powerful tool like Matlab. There you find explanation for indexing a vector, what can be useful together with a for loop.
In a second step you can search in the documentation for specific terms:
docsearch add
Finally you could be looking for a command, which adds the elements of an array. Be aware, that Matlab and its documentation has been designed to solve such standard tasks as efficient as possible.
