1. **State the problem:** Solve the equation $x + 1.29 = 3.78$ for $x$. 2. **Formula and rule:** To isolate $x$, subtract $1.29$ from both sides of the equation. This uses the subtraction property of equality which states that if you subtract the same number from both sides, the equality remains true. 3. **Perform the subtraction:** $$x + 1.29 - 1.29 = 3.78 - 1.29$$ 4. **Simplify both sides:** $$x = 3.78 - 1.29$$ 5. **Calculate the right side:** $$x = 2.49$$ 6. **Final answer:** The solution to the equation is $x = 2.49$. This means that when $x$ is $2.49$, the original equation holds true.