Lens – Calculate the focal length based on the distance between CCD sensors and object patterns

I received a question asking about the focal length of a lens when the distance (center to center) of the CCD pixels in a digital camera is 10 μm. The object is an object with stripe pattern (distance between two strips is 1 mm). The distance from lens to object is 3000 mm. How can I calculate the focal length?

I am very new in Computer Vision and can only understand the basic concept of calculating the focal length if the object size or size is known.