What the standard asks
Texas has no separate grade 9 math standard set. Algebra I is the grade 9 course. A.2(B) asks the student to write linear equations in two variables in various forms, including y = mx + b, given one point and the slope, or given two points. A.2(A) is the sibling skill for domain and range of a linear function. It is not the writing step.
A two-minute check at home
Give them the points (1, 4) and (3, 10). Ask for the equation. They should find slope 3 and write y = 3x + 1. If they can plot the points but cannot write the equation, they are not done with A.2(B).
What it leads to next year
Geometry and Algebra II both assume this writing step is automatic. The Algebra I TEKS devote four of their twelve knowledge statements to linear functions, equations, and inequalities, more than any other topic in the course.
This explainer does not claim that $100 for 100% lessons cover A.2(B). It explains the Texas standard so you can check it yourself.