How do I set a variable in the parent shell, from a subshell?
a=3
(a=4)
echo $a
The whole point of a subshell is that it doesn't affect the calling session. In bash a subshell is a child process, other shells differ but even then a variable setting in a subshell does not affect the caller. By definition.
Do you need a subshell? If you just need a group then use braces:
a=3
{ a=4;}
echo $a
gives 4 (be careful of the spaces in that one). Alternatively, write the variable value to stdout and capture it in the caller:
a=3
a=$(a=4;echo $a)
echo $a
avoid using back-ticks ``, they are deprecated, can be difficult to read and are known to cause issues in certain circumstances.