Blank Box inside of math symbols - JavaScript

Hi all,
We're making a survey that has some math questions in it. We're using MathJax to code and format the math questions, but we cannot seem to get the blank spaces exactly where they need to be. We want to create a question that would have a blank space under a square root sign or within a fraction, like this:
And we want a question where we have blanks within a vector matrix, like this:

We are currently using some JavaScript to add text before or after blanks and this works fine, but how can we get the blanks to fit within some of the operators? Is there a code to move entry boxes on a form question with more control or to anywhere on the page?
Thanks!