for any right triangle with legs length a and b and hypootonuse length c (hypootnuse is longest side aka one oposite right angle)
a²+b²=c²
so
we have c=hypotonuse=17
a leg=a=12
other one=b=x
so
12²+x=17²
144+x²=289
minus 144 both sides
x²=145
sqrt both sides
x=√145 or about 12.04