Because the ladder, the ground, and the side of the building form a right triangle, you can use the Pythagorean Theorem:

$\displaystyle{{5}}^{{2}}+{{12}}^{{2}}={{L}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle{25}+{144}={{L}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle{169}={{L}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle{L}={13}\usepackage{cmbright}\Huge\$