还是二项式定理

(x+y)^n = x^n + C(n,1)*x^(n-1)*y + C(n,2)x^(n-2)*y^2 + C(n,3)x^(n-3)y^3 + C(n,4)x^(n-4)y^4 + ……

式P: C(n,2)x^(n-2)*y^2 = 168
式Q: C(n,3)x^(n-3)y^3 = -70
式R: C(n,4)x^(n-4)y^4 = 35/2

C(n,2) = n(n-1)/2
C(n,3) = n(n-1)(n-2)/6
C(n,4) = n(n-1)(n-2)(n-3)/24

P/Q = -12/5 = 3(x/y)/(n-2)
Q/R = -4 = 4(x/y)/(n-3)
化简后
-4/5 = (x/y)/(n-2)
-1 = (x/y)/(n-3)

两式再做比值运算
4/5 = (n-3)/(n-2)
所以
n = 7

C(7,2) = 21
C(7,3) = 35
C(7,4) = 35

原 P Q 式分别变为
x^5 y^2 = 8
x^4 y^3 = -2

第一个式子两端同时 3 次方，第2个式子两端同时2次方
x^15 y^6 = 512
x^8 y^6 = 4

两式相除
x^7 = 512/4 = 128
x = 2
代入至 x^4 y^3 = -2 中

2^4 y^3 = -2
y^3 = -1/8
y = -1/2

综上所述
x = 2, y = -1/2 , n =7