HOAc		H+   +	OAc-
K = 1.8x10-5		[HOAc] = 0.020 M

	[H+][OAc-]		[x]
1.8x10-5 =		=
	[HOAc]

Since we are calculating [H⁺] let's represent its concentration with x.

What should we use to represent the concentration of acetate ion, [OAc^-]?

0.020 - x   x   0.020