App for "Minds of Modern Mathematics" doesn't work?
Leaving a review is generally the last way to get assistance since the developer and Apple do not reply to those - I resort to a bad/stinging review only after exhausting support from the vendor (or more than a month passes from the request for initial help or explanation.)

In all cases, Apple requires the developer to list a web site for each app. In the minds of mathematics case, since this is a free app, there doesn't appear to be a dedicated support, but if you click through to the developer site:


http://mindsofmath.com there is an about page


It has an email address, so I would start with mailing them asking how to reach the developers support. In this case, your request isn't really best asked to the developer since you are having an issue installing it. You could ask them - perhaps they need to change the devices that the app supports.

In this case, it's installation and Apple should be contacted:


https://support.apple.com/en-us/HT201398
https://support.apple.com/en-us/HT204084


The second article seems to have the best info - if you can't even get that app to install - http://reportaproblem.apple.com/ which seems to work best when you reach it from the HT204084 support page.