Find the value of the 2-form $\displaystyle dx \ dy + 3dx \ dz $ on the oriented triangle with vertices $\displaystyle (0,0,0), \ (1,2,3), \ (1,4,0) $.

I drew the projection of the triangle on each of the coordinate planes. Now what do I do? Do I compute the area of the triangle on the $\displaystyle xy $ plane?

Because $\displaystyle \text{flow} = A dy \ dx + B dz \ dx + C dx \ dy $