How to have executable code in a markdown cell

Question

I'm trying to have a markdown cell that calls a variable in previous code ran, for example:

Code cell: a = 0.00234...

Markdown cell: so, a is (insert code to display exact number)

I have used the nbextension "Python Markdown" which utilizes double curly brackets {{a}}

However, that extension does not work, and yes my notebook is trusted. Please help if you can. Thank you for your time.