Uniform deceleration. A car brakes suddenly & skids to a stop.

Travelling at 18ms^-1 brakes are applied & car skids in a straight line to stop. Coefficient of friction is 0.4. How far does car travel before stopping?
OK, so,
F = μ N but N = mg

so F = μmg
& F = ma
=> μmg = ma

a = μg = 0.4 x 9.81 m s-2
a = 3.924 ms-2

The next bit should be easy, so why can't I do it?
Could you help find the answer please?

g = 9.8 m/s^2

Coefficient of friction is 0.4

9.8 x 0.4 = friction acceleration = 3.92 m/s^2

0.5v^2 = a x distance

0.5 x 18^2 = 162

162/3.92 = 41.32 m