917.254860 Craftsman riding mower won't mow?

We just replaced the belts and the blades and that didn't fix my problem. While cutting the belt seems to make a err err noise, I assume the belt is slipping when it gets in some heavier grass or short already cut grass. Seems there is a lot of play in the belt. I've checked several times and it is the right belt. Looks to me that the blades are turning very slowly as well.What could be wrong? Are the pulleys worn out, is it a bad mandrel(s)? How do i tighten up the belt? Help my grass is now a jungle because I've spent 2 weeks trying to get my mower operational.