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If we take West to be the negative direction, then the initial velocity is -15 m/s and the acceleration (which points East) is +7.0 m/s².
After t = 4.0 s, the helicopter's velocity is
-15 m/s + (7.0 m/s²) (4.0 s) = +13 m/s
or 13 m/s [E].
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