Find the length of side BC in triangle ABC

The correct answer is 5

Using Pythagorean theorem
$AB^2$ + $BC^2$ = $AC^2$
∴ $12^2$ + $BC^2$ = $13^2$
∴ $BC^2$ = $13^2$ - $12^2$
∴ $BC^2$ = 169 - 144 = 25
∴ BC = 5