You have y=x^2/(x-4)^2

Rewrite as y(x-4)^2=x^2

Now, differentiate implicitly wrt time.

2y(x-4)x'+[(x-4)^2]y'=2xx'

Solve for y' and we get:

y'=[(2x-2xy+8y)/(x-4)^2]*x'

Now, enter in x, y, and x' and find y'

Remember, x' is decreasing so it is -2.4