Suppose you increase your walking speed from 4 m/s to 13 m/s in a period of 1 s. what is your acceleration? m/s2

Well the formula for a is a= { vf- vi} ÷ t
where vf is final velocity vi is initial velocity and t is time
thus substituting in the equation you get
a= {13-4} ÷ 1
= 9 m/s2 in the direction of Motion(acceleration is a vector)