If the remainders of the polynomial when divided by and are 7,3 then the remainder of when devided by is

The correct answer is: -2x+5

Dividend = Divisor  Quotient + remainder
f(x) = g(x)  q(x) + r(x)
f(x) = (x+1)  q1(x) + 7
f(-1) = 7
f(x) = (x-1)  q2(x) + 3
f(1) = 3
f(x) is divided with 
Let remainder be ax + b and quotient  be q3(x)