Oh itz ok.........
SOLUTION:-
I think u know this..............
(p x a) 2 + ( p . a)2 = | p |2 + | a |2
--------eq1---------if not ,try to prove it...if u can't get it i will post the proof for this relation also.
substituting the known values in eq1 we get.........
( b )2 + ( 0 )2 = | p |2 + | a |2
| b | 2
-------- = | p |2
| a |2
Taking square root,we get
| p | = +_ | b |
-----------
| a |
Note :-
+_ is plus r minus