Explicație pas cu pas:
N=(7n+3)(n-6n)/5 =>
N=(7n+3)*(-5n)/5 =>
N=-n(7n+3)
n€Z => n(7n+3)€Z => N€Z
N=n(7n+3-2n+2)/5n =>N=n(5n+5)/5n =>
N=5(n+1)/5=>N=n+1
n€Z => n+1€Z => N€Z
