hi i am new here & pardon me if i am wrong..

i just need some help n this some which i hav been trying 2 solve i dont hav anyother book 2 refer..& its too late 2 ask som1 4 der book...

Q. P is the point (1,1,-1). find the equation of the plane passing through P and at right angles to OP.

ans. x+y-z=3

now i dont know who did that came because there aren't any similar solved example & i have tried all the formulas i know....i'll be really thankful if anyone can help me..

& its probably not a lengthy problem since its a 12th grade sum...!