Quick and easy...
The formula is sin(a) = (T-D)/W where a = angle of climb, T = total thrust, D = drag and W = aircraft weight.
For these questions at ATPL level you assume L = W so you have:-
W = 50000kg x g (10 m/s/s) = 500000 N;
L = W = 500000 N;
L/D = 12 so D = 500000/12 = 41667 N (5 s.f.);
T = 4 x 21000 = 84000 N
Plug all of the above into the formula and you get sin(a) = 0.085 (2 s.f.). Multiply this by 100 and you get the climb gradient as a percentage:-
Climb gradient = 8.5 %.
Hope this helps,
V1R