How Does Changing String Tension Affect Frequency in a Physics Problem?

[wentwork882]

Two strings are adjusted to vibrate at 260 Hz. One string is tightened then 3 beats per second is head when the string vibrate at the same time. What is the new frequency of the tightened string.
I am using F_{beat=F1-F2; so wouldn't the answer just be 263 Hz}

 

[Matterwave]

Well, can you explain why 263 Hz, and not 257 Hz?

 

[wentwork882]

The tension increased so the frequency must be greater than the F_{1. But when F2 is isolated the result would be 260-3 = 257hz}