Not sure what "factor theorem" refers to, but one theorem it might be another name for could be the polynomial remainder theorem. It says that a polynomial
![p(x)](https://img.qammunity.org/2019/formulas/mathematics/college/aafakvej27siqwywyuafydxisj3fa5ghh9.png)
, when divided by a linear binomial
![x-c](https://img.qammunity.org/2019/formulas/mathematics/college/egmygri9mkn2zzvw26j41z408kdzmmq93m.png)
, leaves a remainder whose value is
![p(c)](https://img.qammunity.org/2019/formulas/mathematics/college/k9wibqf807mkmib4yqqhe85rrio909j9ju.png)
. If the remainder is 0, then
![x-c](https://img.qammunity.org/2019/formulas/mathematics/college/egmygri9mkn2zzvw26j41z408kdzmmq93m.png)
is a factor of
![p(x)](https://img.qammunity.org/2019/formulas/mathematics/college/aafakvej27siqwywyuafydxisj3fa5ghh9.png)
.
In this case, for
![v+5=v-(-5)](https://img.qammunity.org/2019/formulas/mathematics/college/ifjtjb4o6y00rqnetxmpyhknsi8y273zun.png)
to be a factor of
![p(v)=v^4+16v^3+8v^2-725](https://img.qammunity.org/2019/formulas/mathematics/college/1e49m35rz5xsgdjvr8xo2swqkko7ttbn0x.png)
, we need to check
![p(-5)=(-5)^4+16(-5)^3+8(-5)^2-725=-1900\\eq0](https://img.qammunity.org/2019/formulas/mathematics/college/yhs5k0x3o5azlxfkq5tov6xs4vwl8ok222.png)
So
![v+5](https://img.qammunity.org/2019/formulas/mathematics/college/224e9057r6s69tmzgaobbtlnsld4bkhxpx.png)
is not a factor of
![v^4+16v^3+8v^2-725](https://img.qammunity.org/2019/formulas/mathematics/college/gtdfaur3mutybhccdhbdef8gejxbx1poku.png)
.