What mathematics concepts or principles did you apply to come up with the solution of each equation? explain how you applied these.
me

You need to look at the question again, and notice the words "... did you ...".
From those words, it's clear that the question comes AFTER you have solved
several equations, and now, this question is asking you how you solved them.

The only way for YOU to answer this question is to solve the equations first.

And if you want someone ELSE to tell the math concepts and principles that
are used to solve them, then you have to show him what the equations are.