Bad drivers

Classical Mechanics Level 2

You are driving a car at $30\text{ m/s}$ when a bad driver traveling at a constant speed of $20\text{ m/s}$ swerves into your lane $20$ meters ahead. As soon as you see the car in front of you, you begin to brake. What is the smallest deceleration needed, in $\text{m/s}^2,$ to avoid rear-ending the car? You may assume your acceleration is constant.