Two-step equations

The expression 3x + 4 was built in a specific order: first multiply x by 3, then add 4. To solve, you take the building apart in reverse order: undo the add first, then undo the multiply. That is why removing the +4 comes before dividing by 3. Untangling a knot is easier when you undo the last knot you tied first.

Isla's mistake was jumping straight to the division, splitting 19 by 3 while the +4 was still attached to the 3x. The 4 belongs to the whole expression 3x + 4, not to the x alone, so dividing first leaves a result that is still off by the hanging 4. She skipped a step, and the missing step shows up in the answer.

The fix is two balanced moves. First subtract 4 from both sides: 3x = 19 - 4 = 15. Then divide both sides by 3: x = 5. Each move mirrors the other side, and the equation stays level at every line. The moves are the same as a one-step equation: you just do two of them in the right order.

Check by substituting: 3 * 5 + 4 = 19, so the answer fits the original equation. The two-step order rule is reliable: addition or subtraction first, multiplication or division second, because that is the reverse of how the expression was assembled. Work backwards through the operations and the x frees itself. Build it, undo it in reverse, check it: the loop closes on the original equation.