1. **State the problem:** A rocket ship starts from rest and accelerates constantly at $4\ \text{m/s}^2$ to the right. We need to find its velocity after $3\ \text{s}$. 2. **Formula used:** Velocity under constant acceleration is given by $$v = v_0 + at$$ where $v_0$ is the initial velocity, $a$ is acceleration, and $t$ is time. 3. **Given values:** - Initial velocity $v_0 = 0\ \text{m/s}$ (starts from rest) - Acceleration $a = 4\ \text{m/s}^2$ - Time $t = 3\ \text{s}$ 4. **Calculate velocity:** $$v = 0 + 4 \times 3$$ $$v = 12\ \text{m/s}$$ 5. **Interpretation:** The velocity after 3 seconds is $12\ \text{m/s}$ to the right (positive direction). **Final answer:** $$\boxed{12\ \text{m/s}}$$