x + 4y = 21.....................................(1)
4x - y = 16 Multiply this by 4:-
16x - 4y = 64 ...................................(2)
Add (1) and (2):-
17x = 85
x = 5
Plug this into equation (1):-
5 + 4y = 21
4y = 16
y = 4
Answer is x = 5, y = 4.