lens – Calculates the focal length using the distance between the sensors in the CCD and the object pattern

I was asked a question that asked to find the focal length of a lens when the distance between (center to center) CCD pixels in a digital camera was 10 μm. The object is an object with a pattern of strips (the distance between two strips is 1 mm). The distance between the lens and the object is 3000 mm. How can I calculate the focal length?

I am very new to computer vision and can understand the basic concept of calculating the focal length only when the size of the object or the size of the image is known.