Answer
$\sqrt 2$
Work Step by Step
We have
$$
-\sqrt{18}+2\sqrt{8}= -\sqrt{2\times3^2}+2\sqrt{2\times2^2}\\
=-3\sqrt 2+4\sqrt 2
.$$
Now, we simplify by factoring out $\sqrt 2$ and then combine the like terms as follows:
$$
-3\sqrt 2+4\sqrt 2=(-3+4)\sqrt 2=\sqrt 2
.$$
