interest rate swap

It is the missing figure. There is a total saving to be made of (6 + SOFR+0.5) - (5 + SOFR+0.8) = 0.70, or 0.35% for each to them. Therefore X must end up paying 6 - 0.35 = 5.65% As a result of the swap, X will pay SOFR + 0.8, and will receive SOFR from Y, which so far means they are paying 0.8%. So for them to end up paying 5.65% it must mean that they will pay Y 5.65 - 0.8 = 4.85%