Finding a cheaper implementation for XNOR logic gate
This is the truth table:


As you can see, it shows you how will the output react to different combinations of inputs. The "BOOLEAN" section shows the conditions for which that particular combination yields an output of 1 or TRUE. In other words, that is the algebraic expression for a TRUE output.

Now if any of these BOOLEAN expressions is 1, the output is also 1, so when you write f, you will put a + which signifies OR between them and obtain the rule for the circuit's behaviour. 

Next you will simplify the equation. Take a look at this page.
Find the appropriate rule and then use it to get your result. Don't worry, it should be headache free.