see the attached figure to better understand the problem
we have that

Step 1
<u>Find the value of AC</u>
we know that
in the right triangle ABC

substitute the values in the formula

Step 2
<u>Find the value of BC</u>
we know that
in the right triangle ABC
Applying the Pythagorean Theorem

substitute the values

Step 3
<u>Find the value of BD</u>
we know that
in the right triangle BCD
Applying the Pythagorean Theorem

substitute the values


therefore
<u>the answer is</u>
the length of BD is 11.93 units