Answer
$x = 100 m$
Work Step by Step
$d = \frac{v^2 - v_0^2}{2a}$
Since car is stopping, $v = 0$; $v_0 = 30m/s$
$d = 60 - 30 \times 0.5 = 45m$
$a = \frac{-(30^2)}{2\times45} = -10m/s^2$
For $v_0 = 40m/s$, we will have following
$d_1 = 40 \times 0.5 = 20m$
$d_2 = \frac{-(40^2)}{2\times(-10)} = 80m$
Here $d_1$ is distance traveled because of driver's reaction time and $d_2$ is distrance traveled while stopping, these amounts to final answer
$d = d_1 + d_2 = 100m$
