Question 211795
We use cosine law to solve this problem:
{{{b^2=a^2 + c^2 -2ac*cos B}}}
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Solving for cos B,
{{{cos B=(a^2 + c^2 - b^2)/(2ac)
        =(7.8^2 + 3.9^2 - 4.2^2)/(2*7.8*3.9)=0.9601}}}
B=arc cos 0.9601=<u>16.2 degrees</u>