The text command is nice.
$$
k_1,k_2=H\pm\sqrt{H^2-K}
\qquad
\text{where}
\qquad
\left\{
\aligned
K&=\frac{eg-f^2}{EG-F^2}\\
H&=\frac{Eg-2Ff+Ge}{2(EG-F^2)}
\endaligned
\right.
$$

Interspersing text happens often in math mode.
$$
\gather
g=\det(g_{ij})\tag3--2\\
g^{kl}=(k,l)
\text{ entry of the inverse matrix of }
(g_{ij})\tag3--3
\endgather
$$

\bye