Final answer:
The value of 'k' cannot be determined from the given information because the question lacks necessary context or details that explain how 'k' is related to the provided equation y = ab^x.
Step-by-step explanation:
The question is asking us to find the value of a constant 'k' given that the other two constants, 'a' and 'b', have already been determined in a mathematical expression of the form y = ab^x. The values mentioned in the question are a = 4 and b = 6.25. However, there's no clear indication of how 'k' relates to 'a' and 'b', or what its role is within the given expression, as there is no use of 'k' in the provided functions which suggests there might be a part of the question missing. Without additional information or context, it's impossible to provide a correct value for 'k'.