f(x) = px⁷ + qx³  + rx - 4 = x(px⁶ + qx²  + rx) - 4

set A = px⁶ + qx²  + rx

=> f(x) = x(A) - 4

with x = -7

=> f(-7) = -7(A) -  4 = 3       => A = -1

with x = 7

=> f(7) = 7(A) - 4 = 7(-1) - 4 =11