Answer:
-3.5 m/s²
Explanation:
We know that :-
[tex]\longrightarrow [/tex] Stopping distance = u²/2(-a)
[tex]\longrightarrow [/tex] 63m = (21m/s)² / -2a
[tex]\longrightarrow [/tex] a = - 21 * 21 / 63 * 2 m/s²
[tex]\longrightarrow [/tex] a = - 3.5 m/s²
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