How to write an equation for functions

Functions main problem is to find the missing variable. A function is an equation in which all of the variables are dependent. For example: if you buy 2 bags of oranges and you pay ten dollars for them but the amount of money you pay for the oranges would change each time you add another bag of oranges and how much you owe for the oranges is the dependent variable. So writing a equation would mainly look like this:

Hope this helps!

e) none of the above because the measure of angle cba is 101°