1. **State the problem:** Elizabeth's subtotal is $x$ dollars. A tax of 6% is added, and then a tip of 21% is applied to the total after tax. 2. **Write the formula for tax:** Tax amount is 6% of $x$, which is $0.06x$. 3. **Calculate total after tax:** Total after tax is subtotal plus tax: $$x + 0.06x = 1.06x$$ 4. **Write the formula for tip:** Tip is 21% of the total after tax, so tip amount is $$0.21 \times 1.06x = 0.2226x$$ 5. **Calculate total amount paid:** Total paid is total after tax plus tip: $$1.06x + 0.2226x = 1.2826x$$ 6. **Final expression:** The total amount Elizabeth paid is $$\boxed{1.2826x}$$ dollars. This means she pays 128.26% of the original subtotal including tax and tip.