Answer:
a = 15
Step-by-step explanation:
Substitute b = 12 , a = 9 into the equation
12 = [tex]\sqrt{a^2-9^2}[/tex]
12 = [tex]\sqrt{a^2-81}[/tex] ← square both sides
144 = a² - 81 ( add 81 to both sides )
225 = a² ( take square root of both sides )
± [tex]\sqrt{225}[/tex] = a , then
a = ± 15
a > 0 , so a = 15
