A girl stands 2.8 m in front of a plane mirror…

Step 1. The walk. In $4\ \text{s}$ she walks $0.3\ \text{m/s} \times 4\ \text{s} = 1.2\ \text{m}$. She is now $2.8\ \text{m} - 1.2\ \text{m} = 1.6\ \text{m}$ from the mirror.

Step 2. The image. It is $1.6\ \text{m}$ behind the mirror, so she is $1.6\ \text{m} + 1.6\ \text{m} = 3.2\ \text{m}$ from it.

Step 3. The speed. Each second she gets $0.3\ \text{m}$ nearer the mirror, and so does the image from the other side. Together: $0.3\ \text{m/s} + 0.3\ \text{m/s} = 0.6\ \text{m/s}$.