Well if it's a right triangle, one angle is 90. You said another is 40, and all three angles of a triangle should equal 180. Is side A the hypotenuse or one of the other sides?
Here's the steps, but I don' know offhand how to do each of them:
If you know two angles and any side:
- Use the Sum of the Angles with the two angles to find the third angle
- Use the Law of Sines and plug in the values for the two angles and the side
- Solve for the side
- Use the Law of Sines with an angle, the side opposite it, and the angle opposite the side you still don't know to find that side