Answer
$136^{\circ}$
Work Step by Step
The largest angle in the triangle is opposite to the longest side
From figure, largest angle is A.
using law of cosines
$a^2=b^2+c^2−2bc\cos A$
$cos A=\frac{b^2+c^2−a^2}{2bc}$
$cos A=\frac{24^2+31^2−51^2}{2\times 24 \times 31}$
$cos A=−0.715$
$A=cos^{-1}(−0.715)$
$A=136^{\circ}$
