In a right triangle, a and b are usually used for the legs and c for the hypotenuse.
If that's the case, then you're looking to find the angle opposite the 900 leg.
So relative to angle B, you know the opposite and adjacent sides: 400 (adjacent) and 900 (opposite).
Remember the Sine, Cosine and Tangent (often shortened to sin, cos and tan) functions? You'll know that tan function will work based on the known sides.
tan B = opp / adj
tan B = 900 / 400
tan B = 2.25
tan-1 = 66° (angle B)
Learn more about finding angles when two sides are known at Teacherschoice.com.au and Mathforum.org.