Again, Saperable equation!

Hello.

Am stuck at this problem in saperable equations' section in my book.

**Problem: Solve the differential equation **$\displaystyle \frac{dy}{dx}=x+y$ **by making the change of variable **$\displaystyle u=x+y$.

**My solution:**

I did not see any similar problems in my class' lectures and in the book's section.

All what I can do it is rewrite it as:

$\displaystyle ?=u$.

I put "?" because I do not know how to express $\displaystyle \frac{dy}{dx}$ in terms of u. i.e, expressing it using the suggested substiution.

Any help??