How is writing an equation to represent a situation involving two variables similar to writing an equation to represent a situation involving only one variable?

An equation with only one variable has only one letter used in it, and that letter is usually an "x"

An equation having two variables will have two different letters representing them, usually the letters "x" and "y"

The first type could be the equation 5x^3 - 3x^2 + 6x - 50 = 0

The second type could be (x +y)^2 - 7x^3 + 12x = 58.8

1 equations with only 1 variable are usually much easier to solve than an equation with 2 variables, and you cannot solve the latter unless you have two separate equations containing the two variables.