Standard conditions give temp at FL140 -13C. Find the TAS at 260kts CAS.

Nav computer: Find TAS at 140/-13 CAS 260kt = TAS 322kt
Mach index window -13C. From TAS 322kts read M0.51

Confirm it with the formula: M = TAS/LSS
-13C = 260A
LSS = 38.94 (sq. root 260) = 628kt
322/628 = M0.51