You are walking toward the back of a train that is moving forward with a constant velocity. The train's velocity relative to the ground is 30 m/s forward. Your velocity relative to the train is 1.5 m/s backward. How fast are you moving relative to the ground?
There are 2 different velocities: the train velocity and yours velocity. They're in opposite directions, so one is positive and other is negative. Take the forward direction as the positive:
