An equation that expresses a relationship between two or more variables, such as H=910(220−a), is...

A formula can also be called an equation that expresses a relationship between two or more variables.
The action of finding formulas to describe real-world phenomena is called modeling.
A formula models real-world phenomena means that the formula gives a relationship between the variables that represents the real life situations.
Thus, the complement statement is "An equation that expresses a relationship between two or more variables, such as ${\displaystyle H=\frac{9}{10}(220-a),}$
is called ${\displaystyle \frac{a}{an}}$ formula . The process of finding such equations to describe real-world phenomena is called mathematical modeling . Such equations, together with the meaning assigned to the variables, are called mathematical models ".