A: To do this you have 
to have a routine for parsing and evaluating
your expression. This is a complicated task requiring a clever use
of recursion. You can find such code in Stephen O'Brien (1988),
Turbo Pascal, The Complete Reference. Borland-
Osborne/McGraw-Hill,
Chapter 10. Another, simpler piece of code can be found in 
Michael
Yester (1989), Using Turbo Pascal, Que, Chapter 5.
   I've also written such a function evaluation program myself, and
much of it is based on the ideas in O'Brien with my own corrections
and enhancements. The resulting program is available as fn.exe
function evaluator in the ftp://garbo.uwasa.fi/pc/ts/tsfunc13.zip
package (or whatever version number is the latest). Note however,
that the source code is not included, nor available.
   Tips from Justin Lee (ossm1jl@rex.uokhsc.edu):
 67666 Sep 22 03:00 ftp://garbo.uwasa.fi/pc/turboobj/parstp30.zip
 parstp30.zip Recursive expression TP7.0/BP/VB/C++ parser, 
R.Loewy
An excellent parser is included with all the Turbo Pascal versions
since TP4.0 as part of the MCALC or TCALC spreadsheet 
example
program. See mcparse.pas or tcparse.pas.