This can sounds really confusing but it's not so don't panic!!
So most likely you've been learning about complementary angles and supplementary angles. Complementary angles are angles that add up to exactly 90 degrees (right angles). And then on the other hand, there are supplementary angles that up to exactly 180 degrees (they are angles on flat lines).
Now what does complement mean? And what about supplement?
It's exactly like what it sounds like! The complement of an angle is an angle that combined with the other angle adds up to 90 degrees. While for supplement it's the angles that you combine with the other one to make 180 degrees.
So, if you get what I'm saying here, writing the equations of how they relate should be a breeze!
So if c is the complement to x they should both add up to be 90 degrees so you can just write this:
x + c = 90
And then for the supplement, s, we know that it adds up with x to equal 180 so we can just write this:
x + s = 180
I really hope this helps! ^-^ Knowing this now will help you so much more throughout your school! Understanding supplementary and complementary angles is SUPER important! (for school that is haha) So I would suggest looking up some videos so you can get some practice (I would suggest Khan Academy videos personally!)
