Answer :

Use the law of cosine as the statement suggest you:

[tex]BC^2=AB^2+AC^2-2(AB)(AC)\cos(A)[/tex]

Replace and solve:

[tex]BC^2=19^2+12^2-(2*19*12*\cos(138))[/tex][tex]BC^2=843.87[/tex][tex]BC=\sqrt{843.87}[/tex][tex]BC=29.05[/tex]

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