정답
$y = mx + b$, where $m$ is the slope (rate of change) and $b$ is the y-intercept $(0, b)$.
해설
Slope is given by $m = \frac{y_2 - y_1}{x_2 - x_1}$. Parallel lines share identical slopes ($m_1 = m_2$), while perpendicular lines have negative reciprocal slopes ($m_1 \cdot m_2 = -1$).