Question 641566
The remainder theorem is this:

if you divide f(x) by x-t and you get some remainder 'r', then f(t) = r


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In this case, x+2 is really x-(-2) which matches with x-t. So t = -2


We can use the theorem in reverse to get



f(x)=2x^6-8x^4+x^3-20


f(-2)=2(-2)^6-8(-2)^4+(-2)^3-20 


f(-2)=2(64)-8(16)-8-20


f(-2)=128-128-8-20


f(-2)=-28


So if you divide 2x^6-8x^4+x^3-20 by x+2, then you get a remainder of -28


Since this remainder is NOT zero, this means that x+2 is NOT a factor of f(x)=2x^6-8x^4+x^3-20