Answer: 10y
Explanation:
First and foremost, we should note that: 1 centimeter = 10 millimeters.
Since the length of the line is y centimetres, the expression in terms of y for the length of the line in millimeter will be:
= y × 10
= 10y
For example, if the length is 3cm, it'll then be: = 10y = 10 × 3 = 30 millimeter