The formula for gravitational potential energy is
Ug = mgh where m = mass, g = acceleration due to gravity and h + height
If I understand your question correctly, the ball is hanging 2.1 m from the ceiling, which means that its height is 3.85 m.
In that case, you will have the following equation
Ug = 1.98*9.8*3.85
When you multiply this, you come out with 74.7054 J
If I have misunderstood your question, you should still be able to do the math using the same formula. For example, if the ball is meant to fall from the ceiling and be stopped by the string at 2.1 m from the ceiling, you just substitute 2.1 for 3.85.