Was quizzed over something that wasn't in a video. Any help on what the question means?

Hello Macy! I'm sorry you are frustrated with Khan Academy. When you used the hints did you use all the hints? I did a similar problem of z1 + z2 (see image below) and it walked me through the entire problem. 

Please let me know if you have any further questions, comments, or suggestions. Thank you for posting. 

I saw that, but that wasn't in /any/ of the videos leading up to this unit or lesson. I shouldn't have to get a problem wrong and check an answer to figure what is being asked of me. On top of it, Z clearly has some sort of set value. This means that  if I run into them again with anything other then a 1 & 2 lower exponent...I'm not going to know what to do again. 

z (lower case) is a convention for designating a complex number, often found in higher mathematics, similar to x (also lower case) being used for real numbers in subjects such as Algebra 1.

z with the 1 below it (the 1 is known as a subscript) and z with the 2 below it are ways of specifying multiple complex numbers. Sometimes x is shown this way (with the 1s and the 2s) for multiple real numbers.

Example: Suppose z1 = 2 + 4i and z2 = 3 - i. z1 minus z2 would then be 2 + 4i - (3-i) = 2 + 4i - 3 + i = -1 + 5i.

That helps, thank you so much!