Final answer:
The question seems to require an estimate for f4 using linearity, but the information provided is insufficient to determine an accurate answer without additional context or definition of the function f.
Step-by-step explanation:
The question appears to involve using a linearity formula to estimate f4 given that f2 = 5 and f² = -6. However, without a clear definition of the function f, its operations, and the context in which this problem sits, it is impossible to provide an accurate answer. In general, linearity refers to the principle that the sum of the outputs is equivalent to the output from the sum of the inputs, a principle that holds for linear operators or functions. If f were such a linear function, and by f2 and f² we understand the function evaluated at 2 and the square of the function, respectively, this information alone would still be insufficient to estimate f4 without additional rules or a given pattern.
We might suspect the problem is looking for an application of a formula or theorem from physics or another mathematical concept based on the given context, but the provided information is fragmented and does not lead to a clear mathematical procedure.