Hi,
The factor theorem states that a polynomial f(x) has a factor (x − k) if and only if f(k) = 0
roots -1+4i and -1-4i
f(x) = a(x-(-1+4i))(x -(-1-4i)) = a(x^2 -2x +17)
a = 1
f(x) = x^2 -2x +17
Check productUsing FOIL
F First terms
O Outside terms
I Inside terms
L Last terms