A world cl sprinter can burst out of the blocks to essentially top speed of bout 11.5 m/s in the first 15.0?

A world cl sprinter can burst out of the blocks to essentially top speed of bout 11.5 m/s in the first 15.0 m of the race. What is the average velocity of this sprinter and how long does it take her to reach that speed?? please show the formula u use thank you!