draw a diagram. Here is where implicit differentiation comes in handy.
at the moment in question, the boat's distance x from the dock is 8 ft. The length of rope z is found by
z^2 = x^2+8^2
so
x^2 = z^2-64
x = √(z^2-64)
Now start taking derivatives. The velocity and acceleration of the boat are
x' = z/x z' = zz'/x
x" = [(z'^2 + zz")(x) - (zz')x']/x^2
Plugging in the values
x=6 and z=10 and z' = -5 we get
x' = (10/6)(-5) = -25/3
Since z' is constant, z"=0, so
x" = [(25)(6)-(10(-5))(-25/3)]/36 = -200/7 = -7.41 ft/s^2