Well the belt is listed as an GX20006 which according to Gates is A section x 88 29/32" so that A 89 looks good enough if you are in the USA.
For the rest of the world you should be using an A 87
There are only 2 idlers, a flat one & a V one
laying under the mower with your legs hanging out the front, the flat is in the left & the V is on the right
The belt comes from the engine pulley then goes around the flat then around the V then on to the tranny then back to the engine.
Check you have the belt on the engine pulley & it has not fallen off and the same with the tranny where the belt can pop off & sit between the pulley & the fan.
When you replace the belt you generally need to replace one or both of the plastic idlers .
The other things that can give grief is the hole in the tension arm ( thing the pulleys are bolted to ) flogging out oval cause no one ever lubes it.
The spring not being put back in the right place so not pulling hard enough
A build up of debris under the brake / clutch pedal preventing it from fully dissengaging .
The input shaft has a fine spline which can get stripped so it slips under load .