If this is in fact an Echo SRM 210, then a manual is linked below.
A line trimmer, also known as a string trimmer, is used to trim grass or weeds. The piece of equipment also goes by the name of a weed eater, or weed whacker.
The Weed Eater Electric Trimmer is one of the easiest to use
http://lawnandgarden.manualsonline.com/manuals/mfg/weed_eater/545117526.html?idRes=15826863 This link should let you obtain the manual for detailed instructions with photos.
Weed trimmer line is made from hardened monofilament nylon of varying thicknesses. PA6/66 copolymer nylon is commonly used in weed trimmer line.
How easy is it to replace a spark plug wire on a gas weed eater?
0.080" per 1993 manual
The Weed Eater brand of trimmers have very few spool tangles
You can get them at Home Depot, Lowes, and some small repair shops.
A weed whacker is a gardening tool which is designed to quickly and efficiently cut through weeds, shrubs, and overgrowth. You may also hear a weed whacker referred to as a weed eater or a string trimmer.
According to the Home Depot website there are two best sellers. They are the Weed Eater 200 mph 430 CFM gas blower/vac, model VS2000BV at a cost of $99.00 and the Weed Eater 9" electric line trimmer, model mini trim auto at a cost of $29.96.