The Casio fx-991MS comes with a built-in equation solver. This is useful if you already know the process for solving particular types of equations but do not want to go through the rote algebraic and arithmetical processes. The fx-991MS is able to solve quadratic and cubic equations containing one unknown variable, and linear systems of equations with two or three unknowns.
Use the "MODE" button on your Casio FX-991MS calculator to choose EQN mode.
Determine which type of equation you need to solve. If it has only one unknown variable, you should use the side arrow to move to the quadratic/cubic equation solver screen. If it has two or more unknown variables, use the simultaneous equation screen--it should be the first screen that appears in EQN mode.
Choose an equation format. For single-variable equations, you can choose "2" for a quadratic equation, or "3" for a cubic equation. Recall that a quadratic equation is of the form, ax^2 + bx + c = 0, and a cubic equation is of the form ax^3 + bx^2 + cx + d = 0. For systems of equations, you can choose "2" or "3" to indicate the number of unknown variables. Note that to solve a system of equations with "n" unknowns, you must have at least "n" equations. Also note that the fx-991MS solves only systems of first order, linear equations and not systems of quadratic or cubic equations.
Enter the coefficients of your equation or equations. The calculator will prompt you for the values of the coefficients represented by a, b, c, and d in the generic equation formats. Use a negative variable to show subtraction. For example, for the equation 5x^2 - 7x + 2 = 0, you would enter a = 5, b= -7, and c = 2.
Hit the "=" button to solve the equation or equations. Use the down arrow to scroll through the values of the solved variables.