macro variable - Help!

Hi, All

I have an dataset "test" as following:

Obs Principal Interest Function
1 100 0.25 EXP(-x)

where "principal" and "Interest" are numeric and "Function" is character,

I want to create an macro variable by combining these three variables. My code is

data _null_;
set test;
%let euqation=%scan(Function-(Principal+Interest)=0,1,"=");
run;

I hope &equation to be resolved as EXP(-x)-(100+0.25) but my code isnt working.

Can anyone have an idea how can i do this?

Really appreciate.