Answer:
Minus 9.09090909% decrease
The segment of length x bisects the chord of length 19.2.
The diameter is 24, so the radius is 12.
You have a right triangle with hypotenuse 12, and one leg 19.2/2 = 9.6
9.6^2 + x^2 = 12^2
92.16 + x^2 = 144
x^2 = 51.84
x = 7.2



with that template in mind, let's see.
to the left by 3 units, C = +3
up by 1 unit, D = 1
![\bf g(x)=-x^2\\\\\\ g(x)=-1(1x+\stackrel{C}{0})^2+\stackrel{D}{0}\implies g(x)=-1(1x+3)^2+1 \\\\\\ g(x)=-(x+3)^2+1\impliedby f(x)\qquad \qquad f(-1.5)=-[(-1.5)+3]^2+1](https://tex.z-dn.net/?f=%5Cbf%20g%28x%29%3D-x%5E2%5C%5C%5C%5C%5C%5C%20g%28x%29%3D-1%281x%2B%5Cstackrel%7BC%7D%7B0%7D%29%5E2%2B%5Cstackrel%7BD%7D%7B0%7D%5Cimplies%20g%28x%29%3D-1%281x%2B3%29%5E2%2B1%0A%5C%5C%5C%5C%5C%5C%0Ag%28x%29%3D-%28x%2B3%29%5E2%2B1%5Cimpliedby%20f%28x%29%5Cqquad%20%5Cqquad%20f%28-1.5%29%3D-%5B%28-1.5%29%2B3%5D%5E2%2B1)
and surely you know how much that is.