ok so thats a 45 45 90 triangle so 
45 n
45 n
90 n root 2
hypotenuse (90) is 15 root 2 
so do 15 root 2= n root 2
do solve this multiply by root 2 on both sides
so 15 root 2 times root 2= n root 2 times       
                                              root 2
so 15 times root 4=n root 4
15 times 2 (cuz sqrt of 4 is 2)=2n (cuz 
                                              sqrt of 4 is 2
so 15*2=2n
30=2n
divide by 2 on both sides
15=n
just look at my image its way easier tho