I prefer to think of it as an equilateral triangle which has been bisected - the image above is one of those halves.

Given that an equilateral triangle has angle 60 degrees and (in this case) side 5 then it follows that sideais half the length it was before being bisected, in other words

If thenbcan be found with Pythagoras theorem.

edit: it's even easier if you're allowed to use trig