Force and motion in Newton's laws

[theunloved]

A 1000kg boat is traveling at 90 km/h when its engine is shut off. The magnitude of the frictional force fk between boat and water is proportional to the speed V of the boat: fk = 70V where V is in meters per second and fk is in Newtons. Find the time required for the boat to slow to 45 km/h.

Attempted to solve:
Take upwards as positive for y direction, x + from left to right

From Newton's law 2, we have:
-fk = ma
a = -fk/m = -70Vi / m

(Vi = V initial)

V = Vi + at
---------> t = (V - Vi) /a

t = - m(V - Vi) / 70Vi (h)
with m = 1000kg, V = 45km/h, Vi = 90 km/h

However, my answer is not correct from the book. Can someone point out where I did wrong ? Thanks

 

[DukeLuke]

If the acceleration is based on the object's speed is it really constant?

 

[theunloved]

If the acceleration is based on the object's speed is it really constant?

Oh I got it, since acceleration is not constant, we can't use all the kinematic equations to solve this problem...But how are we going to find acceleration if we only have one equation ?