If the side lengths are a=39, b=31 and c (unknown), then the Triangle Rule is:
a+b>c; a+c>b; b+c>a, so:
39+31>c, c<70; 39+c>31; 31+c>39, c>8.
Therefore 8<c<70, meaning c is between 8 and 70 in length.
To find the length of c exactly, we need the angle C between a and b, then the Cosine Rule applies:
c=√(a2+b2-2abcosC)=√(1521+961-2418cosC)=√(2482-2418cosC)