N in (-oo:+oo)
(-5/7)*n+1 = 21 // - 21
(-5/7)*n-21+1 = 0
-5/7*n-20 = 0 // + 20
-5/7*n = 20 // : -5/7
N = 20/(-5/7)
N = -28
hope it worked out, since i'm not entirely sure.
