Step-by-step explanation:
[tex] {a}^{2} = {15}^{2} - {9}^{2} [/tex]
[tex]a = \sqrt{ {15}^{2} - {9}^{2} } [/tex]
[tex]a = \sqrt{225 - 81} [/tex]
[tex]a = \sqrt{144} [/tex]
a=12 mm
