Now, remember, our initial concentration is 0.2. And our final concentration is very small. It's very small compared to our initial. That's because we have a very weak acid, not a lot of the acid will dissociate. And so our final concentration, our X, will be much smaller than 0.2, our initial concentration. So we can approximate this guy to be equal to X squared over 0.2
because our concentration will change by that much because we see, look, 0.94 and 0.2 and only changed by 0.6.