Question 296392
The missing side is either a leg or the 
hypotenuse.
If it is a leg, then:
{{{15^2  = 9^2 + b^2}}}
{{{b^2 = 15^2 - 9^2}}}
{{{b^2 = 225 - 81}}}
{{{b^2 = 144}}}
{{{b = sqrt(144)}}}
{{{b = 12}}}
The missing side is a leg, and it equals 12