Two-Step Equation
A two-step equation requires exactly two inverse operations to isolate the variable and find the solution.
Definition
A two-step equation needs two moves to solve: first you add or subtract, then you multiply or divide (or the reverse), until the variable is alone. A two-step equation is a linear equation of the standard form $ax + b = c$ that requires two inverse operations to isolate the variable: (1) undo addition or subtraction, then (2) undo multiplication or division, always following the reverse of the order of operations (outer operations first, then inner ones). For $a \neq 0$, the equation $ax + b = c$ has the unique solution $x = (c-b)/a$ over any field, derived by applying the additive inverse of $b$ and then the multiplicative inverse of $a$, and this existence and uniqueness reflect the field axioms.
Example
Solve $2x + 3 = 11$: subtract $3$ from both sides to get $2x = 8$, then divide both sides by $2$ to get $x = 4$. Solve $x/3 - 5 = 2$: add $5$ to both sides to get $x/3 = 7$, then multiply both sides by $3$ to get $x = 21$, and check that $21/3 - 5 = 2$. Over $\mathbb{Q}$, solving $(3/4)x + 1/2 = 5/4$: subtract $1/2$ to get $(3/4)x = 3/4$, then multiply by $4/3$ to get $x = 1$.
Key Insight
Think of the variable as being wrapped in two layers: you peel the outer layer off first, then the inner layer. The solution to a two-step equation is unique in any field of characteristic $0$, but in a field of characteristic $p$ (a prime), the equation $ax = b$ has no solution when $a = 0$ and $b \neq 0$, and every element is a solution when $a = 0$ and $b = 0$, generalizing the "no solution / infinite solutions" cases of the real setting.