Suppose burst times of A, B and C are $T_A,T_B,T_C$ respectively.
According to ques.-
$T_A = 6+x$. (Here $x$ is A's remaining time .)
A ran for 6-time units and got preempted and B got CPU)
This is possible only when $x > T_B$
Now,
$T_B = 2+y$. (Here $y$ is B's remaining time .)
B ran for 2-time units and got preempted and C got CPU)
This is possible only when $y > T_C$
So we have-
$T_C = 4\\T_B=2+y\\y >T_C \ so \ y >4\\T_A=6+x\\
x >T_B$
So minimum value of y can be 5 so minimum value of $T_B=2+5=7.$
Minimum value of x can be 8 so minimum value of $T_A=6+8=14$