Find the length of side a .how do I find the length of side a?A.12B.6C.144D.square root of 306

Find the length of side a .
Sides:
c = 15
b = 9
a = ?
[tex]\begin{gathered} a=\sqrt[]{c^2-b^2} \\ a=\sqrt[]{15^2-9^2} \\ a=\sqrt[]{144} \\ a=12 \end{gathered}[/tex]The answer would be a = 12