Starting with a 3,4,5 triangle, draw a square on each side.

Connect the outer corners of these squares to form a hexagon.

Draw a square on three sides of the hexagon as shown.

Finally connect the outer corners of those squares to form a larger hexagon.

What is the area of the largest hexagon?

As a follow up, if you fancy it, what if the initial right-angled triangle, instead of having legs 3 and 4, it had legs a and b. What is the area of the outer hexagon in terms of a and b?