Final answer:
To write the length of a line in millimeters, multiply the length in centimeters by 10.
Step-by-step explanation:
To write the length of a line in millimeters, we need to convert the length from centimeters to millimeters. Since there are 10 millimeters in 1 centimeter, we multiply the length by 10 to get the length in millimeters. Therefore, the expression for the length of the line in millimeters is 10x (where x is the length in centimeters).