2-form oriented triangle, Differential Forms

[unintuit]

Homework Statement

Find the value of the 2-form dxdy+3dxdz on the oriented triangle with (0,0,0) (1,2,3) (1,4,0) in that order.

Homework Equations

The Attempt at a Solution

I have tried various subtraction of these coordinates and applying them to the formula but the answer is in the back of the book and I fail to see how one could get that answer.
The book does not give one example of how to calculate with 2-forms there is but a brief description and then 4 problems.

This is a problem from Edward's, 'Advanced Calculus: A Differential Forms Approach'

The answer is a negative rational number. I also fail to see how I derive a rational number from this.

 

[wabbit]

This is an unusual formulation, but have you read the first chapter of that book ?