Please teach me a step by step method of using the Casio fx-991ms for equations and polynomial

Press MODE three times until EQN appears on the screen. Select 1 for EQN.

The calculator will ask Unknowns?. Select 2 for two variables or 3 for three variables.
Let's use an example for 2 unknowns:

The calculator will prompt for the coefficients a1, b1, c1, a2, b2, c2, etc. Enter each coefficient followed by the $=$ button.
For the example above, input:
$2 = 3 = 7 = 4 = -1 = 1 =$

Press $=$ again to display the value of $X$. Press $=$ again to display the value of $Y$.
For the example, you will get:
$X = 1$
$Y = 1.666666667$ (which is $\frac{5}{3}$)
The solution is: $\boxed{x=1, y=\frac{5}{3}}$

Solving Polynomial Equations (Equations with "Degree")

This method is for finding the roots of polynomial equations, such as quadratic ($ax^2+bx+c=0$) or cubic ($ax^3+bx^2+cx+d=0$) equations.

Press MODE three times until EQN appears on the screen. Select 1 for EQN.

The calculator will ask Unknowns?. Press the right arrow key $>$ to change to Degree?. Select 2 for a quadratic equation or 3 for a cubic equation.
Let's use an example for a quadratic equation:

The calculator will prompt for the coefficients $a$, $b$, $c$ (and $d$ for cubic). Enter each coefficient followed by the $=$ button.
For the example above, input:
$1 = -5 = 6 =$

Press $=$ again to display the first root X1. Press $=$ again to display the second root X2.
For the example, you will get:
$X1 = 3$
$X2 = 2$
The solutions are: $\boxed{x=3, x=2}$