Perform the indicated operations & simplify.

Add: (919i)+(21+4i)\displaystyle {\left({9}-{19}{i}\right)}+{\left({21}+{4}{i}\right)}
sum =  

Subtract: (919i)(21+4i)\displaystyle {\left({9}-{19}{i}\right)}-{\left({21}+{4}{i}\right)}
difference =