Recall that
v² - u² = 2 a ∆x
where u and v are initial and final velocities, respectively; a is acceleration; and ∆x is the distance traveled (because the skater moves in only one direction).
So we have
(5.8 m/s)² - (8.8 m/s)² = 2 a (6.0 m)
a = ((5.8 m/s)² - (8.8 m/s)²) / (12 m)
a = -3.65 m/s²
