39^2-36^2=(39-36)(39+36)=3*75=3*3*5*5=a^2, so a=3*5=15", the length of the shortest side (5:12:13 is the ratio of the sides)
10^2+24^2=100+576=676=a^2 the length of the hypotenuse, so a=26" the longest side (5:12:13 is the ratio of the sides, making the triangles similar).