How much horizontal force f must a sprinter of mass 47 kg exert on the starting blocks to produce this acceleration? express your answer in newtons using two significant figures?

This statement is missing the first sentence which states the horizontal acceleration.

I found that the desired horizontal acceleration is 15.0 m/s^2.

Now, you must use second law of Newton to calculate the force.

Second Law of Newton: F = m * a

=> F = 47 kg * 15 m/s^2 = 705 N

705 N