What is the area of the circle if XY=5.4mm

If XY is the radius, simply use the formula pi * r^2. In your case this would become pi * 5.4^2 which approximately equals 91.60884.

If XY is the diameter, then the number must first be divided by 2 to get the radius: 5.4/2 = 2.7. Then we can use the same formula as above: pi * r^2. In your case this would become pi * 2.7^2 which approximately equals 22.90221.