have it input stored in a string, than scan for variables, for example:
for(A,1,length(Str1
sub(str1,a,1→str2
if instring("ABCDEFGHIJKLMNOPQRSTUVWXYZ",Str2
Goto 1
End
Disp "no variables"
pause
stop
Lbl 1
Disp "found variable"
Most of the time, you will have only one variable, if you have two, use your graph, so like your example above 250*a+400*b=0, you could change a to x and b to y, giving you 250*x+400*y=0. subtract 400 y on both sides, giving you -400y=250x. Divide 400 on each side, giving you Y=.625x, and it would give you a graph of every possible number.
However, if you only want to find one, the one above would be your best option.
Good Luck,
Mr. Awesomeness