1. Convert sides to the same unit (meters - SI):
l_1 = 0.21dm = 0.021m
l_2 = 42mm = 0.042m
2. Use the perimeter formula to find the third side:
P = l_1 + l_2 + l_3
0.097m = 0.021m + 0.042m + l_3
l_3 = 0.097m - 0.021m - 0.042m
l_3 = 0.034m
3. Convert to CGS units (centimeter-gram-second):
l_3 = 0.034m \times 100cm/m = 5.5cm
4. Convert to FPS units (foot-pound-second):
l_3 = 0.034m \times \frac{3.28084ft}{1m} \approx \frac{11}{200} ft
Answer:
l_3 = 0.034m (SI)
l_3 = 3.4cm (CGS)
l_3 = \frac{11}{200} ft (FPS)