In a right triangle ABC, $\angle$B = 90°. If AC = 13 cm, BC = 5 cm, then AB is ____________.

Given $\angle$B = 90°, AC = 13 cm and BC = 5 cm.
Let's make a right-angle triangle

Let AB= X cm.
Use Pythagoras theorem.
${\mathrm{AC}}^2={\mathrm{AB}}^2+\mathrm{ }{\mathrm{BC}}^2$
${13}^2={\mathrm{X}}^2+\mathrm{ }{5}^2$
$169=\mathrm{ }{\mathrm{X}}^2+\mathrm{ }25$
$169-25=\mathrm{ }{\mathrm{X}}^2$
${\mathrm{X}}^2 = 144$
X = 12
Therefore, the length of AB = 12 cm