how to show/hide a text field when a specific checkbox is checked/unchecked

Looks like you are looking for Conditional Logic which is not available directly in the Contact Form 7 plugin. It can be done via custom programing but you would need to be rather skilled at WordPress development using PHP and/or jQuery.

Some of the premium Form plugins do offer some Conditional Logic including Visual Form Builder Pro, Gravity Forms, and Ninja Forms.

thanks for the info