I thought it might be useful to explain to you why 6.25 cm^2 is the same as 625 mm^2.
One centimeter is equal to 10 millimeters. However, we are talking about square centimeters and millimeters. Imagine you have a square which is 1 cm on each side, so it is 1 cm^2. If you change that 1 cm to 10 mm, then the square is 10mm on each side. That means the area is 10 x 10, or 100 mm^2. If you picture this square, you can see it. The bottom of the square is cut into one row of 10 little mm squares. But, there are 10 rows going up the side. So, there are a total of 100 little mm squares.