Use
a 3-step procedure....
CAP697 page 76
1. TAS 460kts, but ISA -14 ... so correct .....
446kts
2. Calculation of NAM = NGM x TAS/GS = 1200 x 446/486 = 1101 NAM
3. Start mass 50200 kg...from table = 2800 NAM...subtract 1101 NAM = 1699 NAM
Back to table, find 1699 NAM....(1698 will do).....so End Mass 44000kg
Start mass - End mass = fuel required = 50200 - 44000 = 6200kg
Small correction for ISA -14....-0.9%...-55kg, so answer 6145kg