The ladder and the wall form a right triangle.
The length is the hypotenuse = 15.
The base = 2*(15/5) = 2*3 = 6
Use pythagorean theorem to find other side of triangle:
[tex]a^2 + b^2 = c^2 \\ \\ a^2 + 6^2 = 15^2 \\ \\ a^2 + 36 = 225 \\ \\ a^2 = 189 \\ \\ a = \sqrt{189} = 3 \sqrt{21}[/tex]
