how to find roots of nth degree polynomial with Casio fx-991es
How do I find the roots of an nth degree polynomial equation like 5(x^4)-20(x^3)-45(x^2)+34x+23 = 0 using the casio fx-991es? This is a very common calculator being used even in the board exam for engineering in my country..
I can get it to show me all the roots of 1st to third degree equations without a problem by pressing [MODE] then  then it shows me the option of solving a linear, quadratic, and cubic equation; but no option for an nth degree equation. I tried inputting the whole equation, then equating it to 0, then invoking the solve for x function of the calculator by pressing [SHIFT] then [SOLVE]; but it only gives me one root of the polynomial when in fact this polynomial has 3 roots. I know how to solve it with a graphing caclulator or software as well as manually, but I want to know how to make this expensive scientific calculator that claims to have a lot of functions and even is sort of the official calculator for board exams here do it.
edit: I just found something called successive approximations and newton's method, I'm going to check if this works for solving the nth roots..