Form a polynomial f(x) with real coefficients having the given degree and zeros.
Degree: 4; zeros: -1, 2, and 1-2i
since complex roots come in conjugate pairs,
f(x) = (x-(-1))(x-2)(x-(1-2i))(x-(1+2i))
= (x+1)(x-2)((x-1)+2i)((x-1)-2i)
= (x+1)(x-2)((x-1)^2+2^2)
= (x+1)(x-2)(x^2-2x+5)
= x^4-3x^3+5x^2-x-10