To find the missing length when you have lengths A and B, we need more information about the relationship between A and B or what kind of geometry you are working with (triangle, rectangle, etc.).
If you're looking for a specific operation (like adding, subtracting, finding the hypotenuse in a right triangle, etc.), please provide additional context or a specific equation to solve.
If, for example, you are trying to find the length of the hypotenuse (C) in a right triangle where A and B are the two other sides, you can use the Pythagorean theorem:
\[ C = \sqrt{A^2 + B^2} \]
So, if A = 6.2 and B = 3.8:
\[ C = \sqrt{(6.2)^2 + (3.8)^2} \] \[ C = \sqrt{38.44 + 14.44} \] \[ C = \sqrt{52.88} \] \[ C \approx 7.27 \]
If you meant something different, please clarify!