We will use Mersenne's law that states:
[tex]f=\frac{1}{2L}\sqrt{\frac{T}{\mu}[/tex]
Where f is fundamental frequency, T is the tension, [tex]\mu[/tex] is linear density(mass divided by length) and L is the length of the string.
Let us find the linear density:
[tex]\mu=\frac{m}{L}=\frac{5.25}{0.7\cdot1000}=0.0075\frac{kg}{m}[/tex]
Now we just have to plug in all the number in the formula:
[tex]f=\frac{1}{2L}\sqrt{\frac{T}{\mu}}=\frac{1}{2\cdot 0.7}\sqrt{\frac{765}{0.0075}}=228.12$Hz[/tex]
