Introducing simultaneous equations.
I. From Wentworth's New School Algebra (published in 1898), "Simultaneous Simple Equations" chapter, pp. 174-175, p. 177, p. 178 [click to enlarge]:
II. From Core-Plus Mathematics Project, Course 1, "Linear Equations and Inequalities" chapter, pp. 227-228 [click to enlarge]:
III. Extra Credit:
Core Plus Course 1 (9th grade) contains about a dozen simultaneous equations problems, none involving more than two variables, and all with one of the variables isolated on the left side of the two equations as above. Thus, one variable is already "solved for," and the other one, appearing in two expressions that are equal to one another, can be solved for in a few easy steps.
Wentworth's New School Algebra contains many hundreds of simultaneous equations, many involving three variables, some with the variables in the denominators. Solving them involves multiple algebraic manipulations.
Relate this contrast to the amount of explanation given by the Core-Plus textbook (the entirety of which you see here) to the amount of explanation given by Wentworth above for just one of several algebraic methods.