Question: how to find tension in a stretched string?
Answer: Use the velocity equation to find the speed: v=√FTμ=√56.40N3.09×10−4kg/m=427.23m/s.
The tension would need to be increased by a factor of approximately 20. ... 
Use the velocity equation to find the actual tension: FT=μv2=(5.78×10−3kg/m)(427.23m/s)2=1055.00N.
This solution is within 7% of the approximation.