Thorow them hands, this is the solution:
As you can see, we have a right triangle BCD, therefore we can use the Pythagorean Theorem to calculate BD, as follows:
CD = 10/2
CD = 5 cm
CD = Leg 2
BD² = Hyptenuse² - Leg2 ²
BD² = 13² - 5²
BD² = 169 - 25
BD² = 144
Square root at both sides:
√BD² = √144
BD = 12 cm
