What coordinates should I determine if I want to find the area of a surface determined by a double integral

Question

I'm hoping I wrote the title correctly here.

I need to find out the area of the plain domain D, for which the frontier is $\partial{D}: \begin{cases} y=x^2 \\ y=x \end{cases}$

I think it should be solved with the Green-Riemann equation , but I don't really know how.