Area of triangle created from 3 planes

Hey, first year student here and I'm dumb. Need loads of help please!

Ok, given 3 planes pi1, pi2 and pi3 with vector equations **r**.**n1**=0, **r**.**n2**=0 and (**r**-**a**).**n3**=0 respectively, where **a**, **n1**, **n2**, **n3** are given vectors. No 2 planes are parallel and the third plane is parallel to the line, L, given by the intersection of planes pi1 and pi2. Consider the triangle obtained by intersection of all three planes by a plane perpendicular to L. Find a formula for the area and express it only in terms of the scalars: **a**.**n3**, |**n1**x**n2**|, |**n1**x**n3**| and |**n2**x**n3**|.

I've beens stuck on this question for about 12 hours (not consecutively of coures) and still can't figure out the answer...(Headbang) Any help will be appreciated!!!

Thanks in advance,

- TOD