Amixture of 5% disinfectant solution is to be made from 8% and 4% disinfectant solution. how much of each solution should be used if 36 gallons of 5% solution is needed ?

Write a polynomial function f of least degree that has rational coefficients, a leading coefficient of 1, and the given zeros. 3, 4+2i, 1+(sqrt)7 the answer is supposed to be: f(x)=x(^5)-13x(^4)+60x(^3)-82x(^2)-144x+360 what am i doing wrong?