solve for p, given px+4y-2=0, 2x-y+0=0 are perpendicular

[Larry12]

px+4y-2=0
2x-y+p=0

My result is
p=-2x+y

but thats not a numeric value.
the closest i got was either

p=(-4y+2)/x


could anybody PLEASE englighten me on how to do this properly ?
I think I did it in a wrong way.

EDIT:

px+4y-2=0 and 2x-y+p=0 are lines perpendicular to eachother.

 

[pka]

If two lines neither of which is vertical are perpendicular if the product of their slopes is equal to –1. The first has slope (-p/4) and the other slope is 2.