The missing length
is equal to

Here is a diagram of the triangle, with the lengths labeled:
The Pythagorean Theorem states that:

Expanding the squares, we get:

Combining like terms, we get:

Taking the square root of both sides, we get:

We can simplify the radical by factoring the expression under the radical. We notice that all of the terms in the expression have a common factor of
, so we can pull it out:

We can then factor the expression
using the sum-product pattern:

Substituting this into the expression for
, we get:

Finally, we can simplify the radical by taking the square root of each term:
