if we want to check if (x+1) is a factor of x^4 - 2x^2 + 3
just, sub in when x = 1, which is -1 in the dividend
(-1)^4 - 2(-1)^2 + 3 = 2
since, this is not equal to 0, it is not a perfect factor, the remainder would be 2.
It is summarized as such: if dividing by (ax - b), then P(b/a) = 0, it is a factor, if not a factor, than P(b) = R