Question

module vocabulary
constraint
dimensional analysis
equation
equivalent equations
formula
a select choice is an equation that expresses a relationship between certain quantities.

Explanation:

A formula is an equation that shows the relationship between certain quantities. For example, the formula for the area of a rectangle $A = lw$ (where $A$ is area, $l$ is length and $w$ is width) expresses the relationship between these three quantities.

Answer:

formula