The quadratic formula is derived from a quadratic equation in standard form when solving for x by completing the square. The steps involve creating a perfect square trinomial, isolating the trinomial, and taking the square root of both sides. The variable is then isolated to give the solutions to the equation.
So, taking 149.98 and subtracting the non-taxable total, 26.95, you are left with 123.03, which to equal 10.07 would have to be multiplied by 0.0819, hence the sales tax is 0.0819.
