Using Green's Theorem for Vector Fields

[Calpalned]

Homework Statement

Homework Equations

n/a

The Attempt at a Solution

I don't understand how the book went from calculating Green's theorem on $\int _c Pdx + Qdy + \int _{-c'} Pdx + Qdy =$ (1 in the attached picture) to getting (labeled 2) $\int _c Pdx + Qdy = \int _{c'} Pdx + Qdy$

Thank you

 

[fourier jr]

just work out the derivatives $\frac{\partial Q}{\partial x}$ & $\frac{\partial P}{\partial y}$ & use the fact that for a path C, $\int_{-C} P\,dx + Q\,dy = -\int_{C} P\,dx + Q\,dy$