1. Box off the squares (which you did)
2. Solve the area of the squares: (10cm x 10cm = 100cm each square)
3. Solve the area of the rectangle that is left: (you would do 40cm x 10cm since the other 10cm went with the square. The rectangle should end up to be 400cm.
4. Add the areas: 100cm + 100cm +400cm = 600cm
600cm is the answer