We can find b by using the cosine rule of a triangle.
[tex] b^2=a^2+c^2-2ac CosB [/tex]
First step is to plug in the given values in the above equation. So,
[tex] b^2=9^2+5^2-2*9*5*Cos (120) [/tex]
[tex] b^2=81+25-90(-\frac{1}{2} ) [/tex]
=106+45
=151
So, b^2=151
