I can't really tell from your description the exact orrientaion of the triangle. But the known side is either the hypotenuse or one of the two legs of equal length. There are several ways to figure this. Let's start with the pythagorean theorum.
If BC is a leg, then:
If BC is the hyp, then :
The other way to do it is to use the law of sines.
Just set it up and solve like any ratio. The last way -- the taking a test and need to work quickly way -- is to know the 1 to 1 to sqrt2 ratio for this type of triangle and just do the mental math.
If BC is a leg:
If BC is the hyp:
Hope this helps!
P.S. The labels are less important than the orientation, like quick says.