x^2+5x-990=0
x^2+5x=990
x^3+6x^2-985x+1015
=x^3+5x^2+x^2-985x+1015
=x(x^2+5x)+x^2-985x+1015
=990x+x^2-985x+1015
=x^2+5x+1015
=990+1015
=2005