I've been given two planes. P1 = 2x + ay + 4z = 8 and P2 = ax + 4y +8z = 18

First I was asked to find the normal vectors of both planes:

I found N1 = (2,a,4) and N2 = (a,4,8)

Then I was asked "For what values of 'a' will the two planes be parallel?"

This is where I'm stuck, I cannot find a value of 'a' where the two planes are parallel.