Diffeomorphism $\varphi:(0,1)^2\rightarrow U$. $U$ is a parallelogram

Question

Let $U\subseteq\mathbb R^2$ be a open parallelogram with vertices $(2,3), (6,4), (8,6), (4,5)$. Find a diffeomorphism $\varphi:(0,1)\times (0,1)\rightarrow U$.

How can I find such an Diffeomorphism? I tried to do it with polar coordinates but I think it is wrong.

Answer 1

Hint: look at the coefficients in a linear combination of the vectors spanning the parallelogram.