A steel wire in a piano has a length of 0.7 m and a mass of 0.0043 kg. To what tension must this wire be stretched so that the fundamental vibration with frequency of 261.6 Hz?

##T=823.9N##
For the fundamental frequency:
##f=(1)/(2L)sqrt((T)/(m))##
##L## = length
##T## = tension
##m## = mass per unit length.
##sqrt((T)/(m))=2Lf##
##T/m=4L^2f^2##
##T=4L^2f^2m##
##T=4xx0.7^2xx261.6^2xx0.0043/0.7##
##T=823.9N##