#### Answer

\begin{align*}
a_A=2a_B
\end{align*}

#### Work Step by Step

As we know from the equation of kinematics, the final velocity of Car A is given by
\begin{align*}
v_A^2=u_A^2+2a_A(d)\tag{1}
\end{align*}and the final velocity of Car B is from
\begin{align*}
v_B^2=u_B^2+2a_B(2d)\tag{2}
\end{align*}Both the cars start from rest, So, $u_A=u_B$.
Also, both cars cross the starting line at the same time - $v_A=v_B$.
Combining equations 1 and 2, we get
\begin{align*}
2a_A(d)=2a_B(2d)
\end{align*}\begin{align*}
or, a_A=2a_B
\end{align*}