Answer: Approximately 530.66 square millimeters
===============================================
Work Shown:
A = pi*r^2
A = 3.14*13^2
A = 3.14*169
A = 530.66 square millimeters
We can abbreviate "square millimeters" into "square mm" or into "mm^2".
This value is approximate because pi = 3.14 is approximate.