A formula such as K = 2a + 3b + 20 describes a relation between three variables.
When you know two out of three of the variables, you can calculate the third one.
For instance, when a = 10 and b = 5, it follows that K = 2 · 10 + 3 · 5 + 20 = 55.
When you know one out of three of the variables, a relation between the other two
variables remains. For instance, when b = 5, it follows that K = 2a + 3 · 5 + 20 = 2a + 35.
You can plot a corresponding graph.
If you assign to a fixed number of values like b = 0,5,10,15,20,25 then you can plot a family of graphs for as a function of a. For each value of b another graph is plotted, therefore you see six graphs in this case.
In applications such as calculation models you often see several formulas with a number of variables. Sometimes you can reduce the number of variables and the number of formulas by combining some of the formulas.