Which form defines a linear equation in one variable?

1. A linear equation holds one variable raised only to the first power. 2. It can always be rearranged into ax + b = 0 with real numbers a and b. 3. The coefficient a must not be zero, or the variable disappears entirely. 4. A squared term would make it quadratic instead, so that form is ruled out. _Source: OpenStax Intermediate Algebra (CC BY 4.0), Ch 2 "Solving Linear Equations", section 2.1 Use a General Strategy to Solve Linear Equations_