I need to calculate the permeter of a triangle ABC

Side AB 9cm, Side BC 15cm and Angle B 110°

Using the Cosine Rule I Know

b² = a²+c²-2accos B

b² = 15²+9²-2x15x9xcos110°

It is claimed that

b²=398.345

b=20cm approx, a scaled drawing proves this to be the case.

My problem is, I want to make it

b² = 15²+9²-2x15x9xcos110°

b² = 225+81-2x15x9x.3420

b² = 225+81-92.34

b² = 306-92.34

b² = 213.66

b = 14.62 approx which is wrong!!

If I add 92.34 to 306 I get the correct answer but I do not know where I have gone wrong. Can anybody help me?

berniecole