These mower uses a spring load idler arm. There is no adjustment because of this as they are self adjusting. Either belt is worn out, the wrong size, or the spring tension idler arm is frozen up.
Deck belt is a B (5/8) x 143.5".
Drive belt is an A (1/2) x 69.1".
Also to get to the spring mounting point on the deck you must remove the right hand spindle cover. Both covers should be already off but just in case had to mention it. To reattach the spring while the belt is on requires a heavy spring hook tool. Other methods works just not as well and some are quite dangerous.