Calculate three values:-
A = Maximum Zero Fuel Mass + Take-off Fuel = 70200 + 9800 = 80000 kg
B = Maximum Take-off Mass = 78200 kg
C = Maximum Landing Mass + Trip Fuel = 68000 + 7000 = 75000 kg
Lowest value of A, B or C gives the limiting factor - in this case Maximum Allowable Landing Mass.