A Triangle and Two Squares

You are given two squares A and B. The square A has a side length of $a$ and B has a side length of $b$. The left-bottom point of square A is at $(0, 0)$ and the top-right at $(a, a)$. Square B's left-bottom point is $(x, y)$ and top-right is $(x + b, y + b)$. It's guaranteed that square B lies inside square A (may not be strictly inside, can touch too). In other words, $0 \leq x, 0 \leq y, (x + b