The exact opposite of the C in the equation E = MC^ would be darkness. C is the speed of light, so if you take the light out, -C would be equal to darkness.
The antonym (opposite) of the word fantasy would be reality, or truth.
the exact opposite
Broad. The exact opposite would be wide.
Doubling and subtracting 1
The opposite of an imaginative concept would be unimaginative -- dull, boring, repetitive, or common. Similarly a person who is not imaginative would be dull, predictable, or uncreative.
The opposite (imprecisely) would be approximately, or roughly.
I do not belive that the word avenged has an exact opposite, however, the word legitimize would be fairly close. The opposite of getting revenge for a crime, would be to accept that crime as a legal act.
The opposite of a myth would be a fact or reality. Myths are stories or beliefs that are not based on evidence or truth, while facts are verifiable information that is true and supported by evidence.
An antonym or opposite word for myth would be fact. You could also say nonfiction or reality.
The opposite of free enterprise as "capitalism" would be socialism or communism, where market forces are controlled. The exact opposite would be a completely controlled market, for which there is no single word beyond monopoly (control of supply).
This is not an exact analogy, although entrance can be considered the opposite of exit. The "opposite" of minute (small) would be large, big, or huge.
Enemy would be the opposite of friendship. Friendship is when you like and care about each other, and want each other to do well. The exact opposite is true of enemies.