Is this to calculate you car's res. freq?

How do i figure it out, I want to re-design a slot port for my box and want to make it as best as possible for my car.

 

I would just get a meter and run sign waves to figure out which one is louder.

 

my box is tuned to 42hz and was peakin at 48hz

 

Anyone know the formula?

 

I've basically done a lot of reading and came to the conclusion that this would not work out for me due to my port placement being located 2 feet behind my sub. I understand the reasoning is to match up the 180 phase of the wave up with the 0 phase so they can be at the same timming to hit the mic.
so ya. poo