x
x
x


Do so in a way which:

 does not involve string manipulation of the input source code
 is plausibly extensible to a runtime-chosen set of bindings rather than just x
 does not make x a global variable


or note that these are impossible.



See also

 For more general examples and language-specific details, see Eval.
 Dynamic variable names is a similar task.


Input source code is "10 x" , X is locally bound to 3 & 2 and the resulting expressions evaluated.
(10 x /. x -> 3 ) - (10 x /. x -> 2 )
-> 10