97ceae7c-dc38-4574-a7d1-be9893d9bf3d 19 4 5 false false 0 decimal

&[DATE] &[TIME] - &[FILENAME]

Without line

sols x 2^9 - x Solve :

sols x 3 - ≡ x 3 ≡ 21 sys

a.1 x sols 1 el at : 9 - x 2^+ x Solve 1 el

Solve and at don't follow usual SMath rules for evaluation

a.1 x sols 1 el at eval : 3 -

This works only in the case that there are not undefined variables in the equation except x, as you point in your post

With line

sols x 2^b 2^- x Solve 11 line :

sols x b - ≡ x b ≡ 21 sys

a.1 x sols 1 el at : b -

a.2 x sols 2 el at : b

It works

Another way

u sols u 2^9 - u Solve :

x sols x 3 - ≡ x 3 ≡ 21 sys

a.1 x x sols 1 el at : 3 -

a.2 x x sols 2 el at : 3

Here you can see that it is indeed necessary to prevent the direct evaluation of Solve so that at () receives the equalities.