Solve $x^2+(3/4)x-1=0$, leaving the roots in surd form.

Multiply by 4: $4x^2+3x-4=0.x=\frac{-3\pm \sqrt{9+64}}{8}=\frac{-3\pm \sqrt{73}}{8}$. The denominator is $2a=8$ for the cleared equation; keeping $a=1$ from the original line gives the wrong denominator 2, and using a instead of $2a$ gives 4.