Variable Equation.
Equations typically always have (a) variable(s) so it's kind of like asking, "What a word that has letters called?" It's not quite the same, the question is valid. I'm just pointing out that it's a little redundant to say the following, but the best match for what you need would probably be: variable equation.
Chat with our AI personalities
They are called the "solutions" of the equation.
An open sentence.
It is the unknown variable
You cannot plug in a variable, what you do is plug in the value for a variable. If you know the value of the variable in an equation (or formula), the process of replacing that variable whenever it appears in the equation by its value is called plugging in the value for the variable.
Equations can be classified according to the highest power of the variable. Since the highest power of the variable in a linear equation is one, it is also called a first-order equation.