According to the -x value hint, maybe look at x - n values since (-x + n) = (x - n) and see the u that that creates.

Just a quick look:

c287

X = 15, N = 128

(X+N) = 143, U = 2556, (X+N)^2 = 20449

(X-N) = 113, U = 56, (X-N)^2 = 12769

f = 2, e = 31

(31, 1, 7) -{31:1:113:13:100:128}

(31, 1, 8) -{31:1:143:15:128:160}

(-2, 1, 8) -{-2:1:111:14:97:127}

(-2, 1, 9) -{-2:1:143:16:127:161}

|

(31, 1, 0) -{31:1:15:-1:16:16}

(31, 1, 1) -{31:1:17:1:16:20}

(-2, 1, 1) -{-2:1:-1:0:1:-1}

(-2, 1, 2) -{-2:1:3:2:1:7}

287 = 7 * 41, x = 9, n = 8

(x+n) = 17, u = 8, (x-n)^2 = 289

(x-n) = 1, u = 1, (x-n)^2 = 1

The patterns say, look, I, the solution am here! But can we bring it out?