Answer: 9
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Work Shown:
a = unknown = leg #1
b = 12 = leg #2
c = 15 = hypotenuse
Plug those values into the pythagorean theorem and solve for 'a'
a^2 + b^2 = c^2
a^2 + (12)^2 = (15)^2 .... substitution
a^2 + 144 = 225
a^2 = 225 - 144 ... subtracting 144 from both sides
a^2 = 81
a = sqrt(81) .... applying square root to both sides
a = 9