To protect your car from freezing to -20 degree C. how do you make C2H6O2 aqueous solition ?

Supposing C2H6O2 to be something that doesn't ionize:

(20 °C) / (1.86 °C/m) = 10.75 m C2H6O2

(10.75 mol C2H6O2) x (62.0678 g C2H6O2/mol) = 667 g

So your solution must contain 667 grams of C2H6O2 for every 1000 grams of water in the mix.

C2H6O2 can be the formula for several compounds ... You are probably referring to ethylene glycol (automotive antifreeze). You need some kind of constant that describes the effect of this compound on lowering the freezing point of water. OR maybe your book has a table or graph showing this effect. I don't have your book, so I cannot find the constant, or table, or graph. You must do something.