1. An image-forming apparatus comprising: a rotary body; an image-forming unit configured to perform an image-forming operation; at least two sensors configured to read an image formed on the rotary body; a control device configured to: control the image-forming unit to form a test image on the rotary body; control the at least two sensors to read the test image formed on the rotary body to acquire at least two values, each of the at least two values being determined based on a read image acquired by reading the test image with a corresponding one of the at least two sensors; calculate a weighted average value of the at least two values by using weightings that are assigned to the at least two values, respectively, one of the weightings being different from another one of the weightings; and correct the image-forming operation using the weighted average value.