The independent variable is the party.
The dependent variable is the number of tacos Brian will make.
The independent variable is the cause.
Why is Brian making tacos? Brian is making tacos for a party.
The dependent variable is the effect.
What is the effect of the number of people that will be attending the party? The effect will be of how many tacos Brian will make.
Remember:
The independent variable causes an effect on the dependent variable.
I apologize if this is wrong. I hope you have a lovely day/night! :)