Can someone help me find the missing side length!

12

Step-by-step explanation

so what I did is I divided 15 by 5 to get 3 also 9 divided by 3 equals 3 to so you could of also done that.

then i multiplied to 4 x 3 to get the missing side length which equals 12

the answer is 12

Step-by-step explanation:

to find the length we us this formula a²+b²=c²

a²+b²=c²

a²+(9)²=(15)²

a²+81=225

a²=225-81

a²=144

a=√144

a= 12 unit