Final answer:
To calculate EF, additional information is needed to understand how EF relates to DF since just knowing DF=9x-39 isn't enough to determine the length of EF.
Step-by-step explanation:
To find the value of EF given that DF = 9x - 39, we would need additional information regarding how EF is related to DF. If EF and DF are parts of the same line segment, and you need the length of EF given that of DF, then knowing the total length of the line segment ED or the value of x that makes DF a specific length would be necessary. Without this supplementary information, we cannot determine the length of EF. If you come across such additional context within the problem, make sure to apply relevant geometric or algebraic principles to find the required length.