Stuck on fp3 vectors/planes problem

Can someone helped me with the attached question (part a)?

I've found the midpoint of p and p' (2, -1, -1) which the plane passes through (I think). The plane would be perpendicular to the vector PP' - but I'm stuck on how I'd find this normal vector using the cross product?

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I was thinking the plane would be perpendicular to the vector PP'. So once I've found the vector PP', how do I use this to find the cartesian equation of the plane?

Let me ask you a different question.

If you had a plane ax+by+cy=d, say, can you tell me a normal vector for that plane?

PS: Quote if you want a reply - I'll usually see it sooner.

r.n=a.n so n in this case would be n=(a b c )?

Yes.

So reverse the process. You have a normal vector, hence you have possible values for a,b,c. You just need to work out d, and for that you know one point in the plane, that must satisfy its equation.