The time is given as,
t=Ax2+Bx
Differentiate the above expression with respect to t,
1=2Axdxdt+Bdxdt
dxdt=1(2Ax+B)
Again differentiate the above expression with respect to t,
d2xdt2=−1(2Ax+B)2×2Adxdt
a=−2A(2Ax+B)3
Thus, the retardation of the moving particle is −2A(2Ax+B)3.