How does this affine cipher work?
W is 87 in ASCII, so
$$
87a+b\equiv064066\pmod{256256}.
$$

I is 73 in ASCII, so
$$
73a+b\equiv158368\pmod{256256}.
$$

Subtracting, you get
$$
14a\equiv-94302\equiv161954\pmod{256256}.
$$

Unfortunately 14 is not coprime to 256256, so you'll need to use other letters to figure this out. Once you get an equation of the form
$$
ma\equiv n\pmod{256256}
$$
you can solve for $a\pmod{256256}$ and then substitute to find $b\pmod{256256}.$

(In fact, you already have enough to solve with trial and error, but it's nicer to avoid that.)