Answer
The amount of time elapsed before the skater's velocity is reduced to the final velocity is $4.43s$.
Work Step by Step
Kinetic frictional force $f_k$ would decelerate the ice skater's speed at a rate $a$. The formula of $f_k$ is $$f_k=\mu_kF_N$$
Since there is no vertical movement, normal force $F_N=m_{skater}g$
$$f_k=\mu_km_{skater}g$$
According to Newton's 2nd Law, and since $f_k$ opposes the motion, $$-f_k=-\mu_km_{skater}g=m_{skater}a$$ $$a=-\mu_kg=-0.081\times9.8=-0.79m/s^2$$
We have initial velocity $v_0=6.3m/s$ and final velocity $v=2.8m/s$. The amount of time elapsed before the skater's velocity is reduced to the final velocity is $$t=\frac{v-v_0}{a}=4.43s$$
