Even if you know Flexbox you cannot use Flexbox because support lacks. http://caniuse.com/#feat=flexbox The TL;DR: Everyone is doing well except IE which is horribly broken. Only Microsoft's Edge browser has good Flexbox support, but nobody uses Edge, so therefore Flexbox is still no-go. Come back in five years when IE 11 is a distant memory and Edge is popular.
There's plenty of cases (especially in the enterprise, where requirements can be tightly controlled) when non-Edge versions of IE can be safely dropped. I'm working on such a project right now, we use flexbox very extensively.
I am considering flexbox but have to worry about supporting above. Luckily we don't have legacy IE to worry about so am hopeful.
according to this [0] ie 10 and 11 are supported but with "partial support"?