As the question say this problem is a practice of the law of cosines!
The law of cosine: c^2=a^2+b^2-2ab*cos*c for any side a, b and c
The law can also be written as c=sqrt(a^2+b^2-2abcosc)
Now use this formula!(note cos 100 degrees is about 0.8623)
c=sqrt(15^2+16^2-2*15*16*cos100)
solving this we can MN is about 23.75
Round this to the nearest tenths now!
We get 23.8!
Thus the answer is D
