Hi,
How can you find the limit without knowing what a is? Also you have square brackets where they should be round and you should use lower case so you don't bump into reserved symbols.
In[11]:= Limit[de (1 - Exp[-a (r - re)])^2, r -> Infinity,
Assumptions -> a < 0]
Out[11]= ComplexInfinity
In[12]:= Limit[de (1 - Exp[-a (r - re)])^2, r -> Infinity,
Assumptions -> a > 0]
Out[12]= de
