What is the value of m in the figure below? If necessary, round your answer to the nearest tenth of a unit.

A. 6.7
B. 7.5
C. 3
D. 12

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