Answer:
5
Step-by-step explanation:
using Pythagorean theorem:
[tex]a^2+b^2=c^2[/tex]
a and b are sides
c is the hypotenuse
in this case we need a side:
[tex]a^2+12^2=13^2\\a^2 + 144 = 169\\[/tex]
subtract 144 from 169
[tex]a^2=25\\\sqrt{a} ^{2} = \sqrt{25}[/tex]
so a = 5
THE ANSWER IS 5 :)
