Let's draw a rough sketch:
We have a 20 foot ladder that is leaning against a house, height h.
The base of the ladder is 3 feet from the base of the house.
We need to find h.
We can use the pythagorean theorem to find h. So,
[tex]\begin{gathered} 3^2+h^2=20^2 \\ 9+h^2=400 \\ h^2=400-9 \\ h^2=391 \\ h=\sqrt[]{391} \\ h\approx19.8 \end{gathered}[/tex]
