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. That's a very small amount. So now we have this very simple situation in which we simply bring over the 0.2, multiply it by 1.8
times ten to negative five equals X squared. Take the radical, and we get 1.8 times ten to negative five times zero.