Answer
$x= -5, x= -1$
Work Step by Step
Use the completing the square method. $$
\begin{aligned}
7(x+3)^2-8 & =20 \\
7(x+3)^2 & =20+8 \\
7(x+3)^2 & =28 \\
(x+3)^2 & =\frac{28}{7} \\
(x+3)^2 & =4 \\
x+3 & = \pm 2\\
x & = -3\pm 2.
\end{aligned}
$$ This gives: $$
\begin{aligned}
x & =-3-2 \\
& =-5 \\
x & =-3+2 \\
& =-1.
\end{aligned}
$$ The solutions are: $$x= -5, x= -1.$$ Define the following version of the function and plot it. We see that the zeros are where they should be. $$
f(x) = (x+3)^2-4.
$$
