Solve the missing angles

a=15cm b=18cm c=30cm

so If I use $\displaystyle a^2=b^2+c^2-2bc\cdot\cos A$

i get angle A is 42.12 degrees but then B and C don't work out i know I can use the law of sine once I have one angle but I am just asking why doesn't B and C work with the law of cosine?