I am assuming this is a right traingle
if a right triangle has legs legnth a and b and hypotonuse legnth c then
a²+b²=c²
legs are 12 and 16
a=12
b=16
12²+16²=c²
144+256=c²
400=c²
sqrt both sides
20=c
the legnth is 20in, assuming you have a right triangle