If a particle is moving with constant velocity, the x-t graph would be a straight line with a constant slope. The slope of an x-t graph represents the particle's velocity. In the case of constant velocity, the slope remains constant throughout the entire graph.
However, if a particle is moving with constant acceleration, the x-t graph would be a curved line (a parabola). The slope of the x-t graph for a particle with constant acceleration is not constant but changes continuously.
In summary, for a particle with constant velocity, the slope of the x-t graph is constant, while for a particle with constant acceleration, the slope is continuously changing.