Maybe it’s not universal but in school literal equation basically meant there were letters instead of numbers.

It’s the term we use for instance when going from the equation of a line like y=3x+2 to lines in general y=ax+b (a and b in ℝ)

And i agree it’s a lot better to specify to solve for x (because you can solve for anything or have multiple variables).

Although x being a variable, and solving for it would be the most logical assumption.