A method to determine the unknown thermal conductivity k of a material uses two infinitely long fins. Each fin has a square cross-sectional area, with a side length 3 mm. One fin is made of aluminum, with ka = 200 W/mK. The other fin is made of the material with an unknown k value. Both fins are attached (separately) to a base that has a fixed temperature Tb = 1000C. The fins are both exposed to ambient fluid at T* = 250C and h = 5 W/m2K. It is found that at the same value x along the fins, the aluminum fin has a temperature of 750C, while the fin of unknown k value has a temperature of 600C. Find the unknown k value. HINT: Find 'x' for the aluminum fin first, then address the unknown fin.