What is a type expression that contains a variable or variables

An expression for a type of proportionality so like y=kx where y is dependent and x is independent and the y and x will be the variables while k is constant of proportionality This can also be stated k=y/x so to find k you divide dependent by independent Don't confuse this with science trust me you don't wanna do the y/x in science

6+a=12 or 13+b=23 a variable is when there is a letter with something you add, subtract, multiply, and divide something.