|Ok I think I've figured it out.. Can someone check my work? Sorry to be a pest with the triple posts...
$A = \{w,x,y,z\}$
$U = \{w,y\} * A^7$
$S = \{w,y\} * \{w,y,z\}^7$
$|U - S| = |U| - |S|$
$= |\{w,y\} * A^7| - |\{w,y\} * \{w,y,z\}^7|$
$= |\{w,y\}||A|^7 - |\{w,y\}||\{w,y,z\}|^7$
$=...