We are given that the tension of a ligament is proportional to its extension. Therefore, we have from Hook's law:

Where:

Now, we are asked to determine the tension of a string given that is stretched a distance of 0.780 cm:
![F=(150\frac{N}{\operatorname{mm}})(0.78\operatorname{cm})]()
Now, we need to convert the centimeters into millimeters. To do that we will use the following conversion factor:
![1\operatorname{cm}=10\operatorname{mm}]()
Now, we multiply by the conversion factor:
![F=(150\frac{N}{\operatorname{mm}})(0.78\operatorname{cm}*\frac{10\operatorname{mm}}{1\operatorname{cm}})]()
Now, we solve the operations:

Now, we convert the "N" into "kN" using the following conversion factor:

Multiplying by the conversion factor we get:

Therefore, the tension is 1.17 kN.
Now, the elastic energy is given by:

Substituting the values we get:
![U=(1)/(2)(150\frac{N}{\operatorname{mm}})(0.78\operatorname{cm}*\frac{1\operatorname{cm}}{10\operatorname{mm}})^2]()
Solving the operations:

Now, to covert the energy into Joules we need to covert the "mm" into "m". To do that we use the following conversion factor:
![1000\operatorname{mm}=1m]()
Multiplying by the conversion factor:
![U=0.456\text{Nmm}*\frac{1m}{1000\operatorname{mm}}]()
Solving the operations:

Therefore, the elastic energy is 0.000456 Joules.