The speed of an object is the ratio of distance traveled to the time taken. Velocity, on the other hand, is the ratio of displacement to the time taken. Unlike speed, velocity is a vector quantity and has both a magnitude and a direction. In comparison, speed is a scalar quantity and has only the magnitude and no direction.
Speed and velocity will be the same for an object, if and only if the distance travelled is equal to the displacement. This is possible for the case of travel in a straight line.
In the given case, the sprinter runs from a finish line to a line 50 m away in 6 seconds. That is 50 m in 6 seconds and thus has a speed of 50/6 = 8.33 m/sec. Since this is a travel in straight line, distance and displacement will be the same and thus, speed and velocity would be the same.
Hope this helps.