Find the decrypt function...

Question

Hi all.! I am to encrypt a series of numbers using this formula:

E(x)=(w * x + v )% 256

This only works if w is relatively prime to 256, i.e. the only number which divides both w and 256 is 1.

w is equal to: a % 256.
v is equal to: c % 256).
(a and c are known numbers)

So in the above equation w = a and v = c, or w = a%256 and v = c%256 ?
Also, what will be the decryption formula for the above function ?

Thanks in advance...