Answer:
Excel and also OpenOffice Calc have functions to do certain operations on complex numbers. The IMPOWER() function will handle what you want to do. Example: type the following in a cell =IMPOWER("-415+234i",1/3) and the result will be 5 + 6i. If you are using OpenOffice, then a semicolon is used rather than a comma in functions {=IMPOWER("-415+234i";1/3)}
The complex number is formatted as text. There are other functions: IMABS, IMARGUMENT, IMPRODUCT, etc. You can do a help search for Complex Numbers, or one of these functions that I mentioned.