Impossible to answer without brand and model info.
One might learn how to install a 42 inch deck belt on a John Deere lawnmower by looking at their user manual. A better route to this may be to take it to a lawn mower shop for repair.
37x63
42" deck = 71" belt48" deck = 73.8" belt44" SNOWBLOWER uses V-Belt 131"
You will need to order a manual on the mower from the manufacturer (I assume Simplicity).
84 inch
The 66 inch deck belt fits Murray 42 inch and 46 inch models 425303X92A, 425303X92B, 465305X92A, 465305X92B. The width is 11/16 inches. This belt size replaces Murray part numbers 37X112, 37X112MA.
just get new belts they stretch or buy a can of belt grip
The John Deere LX172 38 inch blade deck uses a 65 by 1/2 inch V belt. If you ask for a 65 inch belt at an auto parts or hardware store, there is no need for a multi-numeral part number.
The 42 inch deck belt is 104 X 1/2 inches.
the drive belt is 76 length 1.5 width
5L74 gates
4L 63 inch