Answer
a=15
Work Step by Step
Use the Pythagorean Theorem to find the missing length.
$a^2+b^2=c^2\longrightarrow$ substitute the given values
$a^2+36^2=39^2\longrightarrow$ simplify exponents
$a^2+1296=1521\longrightarrow$ subtract 1296 from each side
$a^2+1296-1296=1521-1296\longrightarrow$ subtract
$a^2=225\longrightarrow$ find the square root of each side
$\sqrt{a^2}=\sqrt{225}\longrightarrow$ simplify
$a=15$
