The boat's initial velocity is:
[tex]v_0 = 20 m/s[/tex]
While the boat's acceleration is
[tex]a=-1.5 m/s^2[/tex]
with a negative sign, since the boat is slowing down, so it is a deceleration. The distance traveled by the boat until it comes to a stop can be found by using the following equation:
[tex]v_f^2 -v_0^2 = 2aS[/tex]
where vf=0 is the final velocity of the boat and S is the distance covered. Re-arranging the formula, we can find S:
[tex]S=\frac{-v_0^2}{2a}=\frac{-(20 m/s)^2}{2(-1.5 m/s^2)}=133.3 m[/tex]