Answer: A) 6.7
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Work Shown:
Use the geometric mean formula to find that BD = 6
BD = geometric mean of AD and DC
BD = sqrt(AD*DC)
BD = sqrt(12*3)
BD = 6
Then use the pythagorean theorem to find the missing side for triangle BCD
a^2+b^2 = c^2
(CD)^2 + (BD)^2 = (BC)^2
3^2 + 6^2 = m^2
m^2 = 45
m = sqrt(45)
m = 6.70820393249937 approximately
m = 6.7 after rounding to one decimal place
