f(x)=x^3+4x^2+3x
I assume you're being asked to factorise this expression or alternatively find the roots of f=0. Since the constant term is zero, there's a factor of x (equivalently, x=0 is a root). So f = x(x^2+4x+3). You can find the roots of the quadratic x^2+4x+3, for example, by the formula, and they are -1 and -3. That corresponds to factor (x+1) and (x+3).