You can solve this using simultaneous equations: You know the elements are Cu, H, N & O. Lets call the unknown coefficients a, b, c, d & e (write this full equation out yourself) Balancing each element in turn (which they must) yields 4 algebraic equations which will yield the ratio of the coefficients to each other.
1) Balancing for Cu gives a = c
2) ....for H gives b = 2e
3) ...for N gives b = 2c + d
4) ....for O gives 3b = 6c + d + e
if you multiply eq 3 by 3 you get 3b = 6c + 3d and joing with eq 4 yields 6c + 3d = 6c + d + e ie 3d = d + e ie 2d = e we now have 2 of the simple equations involving 3 of the coefficients so should be easy to solve from here: try out a value of d = 1 which => e = 2 and so eq 2 => b = 4. Eq 3 => 4 = 2c + 1 ie c = 3/2 and eq 1 => a = 3/2 so the ratio of the coefficients is 3/2:4:3/2:1:2 which is no good as it needs to be all integers and the minimum possible at that. Multiply the ratio throughout by 2 yields: 3:8:3:2:4