Recall that
v² - u² = 2 a ∆x
where u and v denote initial and final velocities, respectively; a is acceleration; and ∆x is the distance traveled.
Then we get
(18 m/s)² - (26 m/s)² = 2 a (52 m)
a = ((18 m/s)² - (26 m/s)²) / (104 m)
a ≈ -3.4 m/s²
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