Answer:
12 centimeters
Explanation:
The width of each guitar pick was = 12 mm
He lined up 10 guitar picks across his desk.
The length of line of his guitar picks = 12 × 10
= 120 mm
Now we will convert 120 mm to centimeter.
10 millimeter = 1 centimeters
120 millimeters =

= 12 centimeters.
The line of his guitar picks was 12 centimeters long.