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.
I use this numbering sequence for manga only, very satisfying to coincide the reading directions with it.