Eh? That's what the actual question says and that's what I've been taught, anyone able to clarify?
Anyway what I've ended up doing is saying if r* was in the plane, it's cross product with either p* or q* (p* x r* etc) would be parallel to p* x q* (p* q* cross product). i.e. they are both normal to the plane formed by p* and q*, making them parallel => making them scalar multiples of each other. Vector parallel still confuse me.
I set [(p* x q*) x (p* x r*)] = 0 vector so that they are parallel and solved for x, getting x=3.