The concept of an opposite is simple. The opposite of a number is a negative. For example, the opposite of 35 is -35.
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It means negatives
There is no such thing as "Straght" in math: the word does not [yet] exist in the English language. It is not easy to define "staright". You start off with a set of primitive mathematical concepts and then define other concepts using them. A straight line is a primitive geometric concept: it is the shortest distance between two distinct points in a plane. [You will probably need to use the concept of straight to define shortest!]
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A math concept is how you solve a math problem or the ideas. You usually have to memorize math concepts.
"key concept" is not a mathematical term . A key concept is an important idea. Two key concepts in math are derivatives and integrals of functions. ( "function" is also a key concept.)