Question 429107
Let h = hypotenuse
then 
{{{ h^2 = 6^2 + 3^2 }}}
{{{ h^2 = 36 + 9 }}}
{{{ h^2 = 45 }}}
{{{ h = sqrt(45) }}}
Next, factor the number under radical with prime numbers:
{{{ h = sqrt(3*3*5) }}}
Pull out "pairs":
{{{ h = 3*sqrt(5) }}}