Use the sine rule to find a (it's the easiest way). If you don't know what it is it's basically making a side and opposite angle equate to another either missing side or angle to find it.
The basic formula is
= (It would flip if you were trying to find an angle, because you put whatever you are trying to find on top)
So here it'd be:
=
Of course you need to rearrange for a, so it would be: