Groups and the Group Axioms · Hard problem

Define $a*b=a+b-ab$ for $a,b$ in $S=\mathbb{R}\setminus\{1\}$; this makes $(S,*)$ a group with identity $0$ (closure, associativity, and inverses can all be checked directly from the formula). Find the value of $x$ for which $(3*x)*5=0$.