s i p (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

LOPE (bound) Z (unknown – in algebra) – we thought that this might be a reference to Jennifer Lopez, the American actress and singer, but an electronic search revealed that there are several Hispanic singers with the surname Lopez – Trini, Pablo, Lisa, Rosa, Brian, Sonia, Ivan, Julio, Kareem, Cesar, Nadia……