Sunita is solving $4x^2-7x-2=0$ and has correctly worked out $b^2-4ac=81$.
Which line should come next?

Here $a=4,b=-7$ and $c=-2$, so the numerator starts with $-b=+7$, the square root of the discriminant is $\sqrt{81}=9$, and the denominator is $2a=8:x=\frac{7\pm 9}{8}=2$ or $-\frac{1}{4}$. Check: 16 − 14 − 2 = 0 and $4\times \frac{1}{16}+\frac{7}{4}-2=0$. Copying $b$ as −7, forgetting to take the square root of 81, and using a instead of $2a$ give the other three lines.