Question 176369
Remember that for right triangles, we have a theorem relating the three sides: Pythagorean's Theorem.  If c is the hypotenuse and a, b are the other two sides, we have that
{{{a^2+b^2=c^2}}}
So if we know that the hypotenuse is 13 and one of the sides (call it a, it doesn't matter) is 12, we have that:
{{{12^2+b^2=13^2}}}
{{{144+b^2=169}}}
{{{b^2=25}}}
So b=5.